can you post your Troubleshooting Information? Help (Alt-H) - Troubleshooting Information (Or press F10 to bring up the menu bar and select Help - Troubleshooting Information.)

kitten2015 said

Yesterday I downloaded the newest version of Moilla Thunderbird. System told me that my present ??? is not compatible with Lightning and to install that update. It ran forever and obviously did not install properly. Today my calendar and all my appointments are gone (it did not warn me that this might happen, so I could not save it). I tried to follow some info but nothing works - I am not able/capable to install a new calendar (does not help that I am not that good with PC's). Please help - My family relies on this calendar being updated on a daily basis. If not possible, where can I find a calendar that I can use?

 Application Basics

   Name: Thunderbird
   Version: 38.0.1
   User Agent: Mozilla/5.0 (Windows NT 6.1; rv:38.0) Gecko/20100101 Thunderbird/38.0.1
   Profile Folder: Show Folder

             (Local drive)
   Application Build ID: 20150608103712
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory

 Mail and News Accounts
   account1:
     INCOMING: account1, , (none) Local Folders, plain, passwordCleartext

   account2:
     INCOMING: account2, , (imap) imap.googlemail.com:993, SSL, passwordCleartext
     OUTGOING: smtp.googlemail.com:465, SSL, passwordCleartext, true

   account4:
     INCOMING: account4, , (imap) imap.googlemail.com:993, SSL, passwordCleartext
     OUTGOING: smtp.googlemail.com:465, SSL, passwordCleartext, true

 Crash Reports
   http://crash-stats.mozilla.com/report/index/bp-485dbd3e-e21f-476f-b88e-f06252121021 (10/20/2012)
   http://crash-stats.mozilla.com/report/index/bp-ce053a2f-cb6b-4944-a21c-1d1d22120406 (4/6/2012)

 Extensions
   Lightning, 4.0.0.1, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}

 Important Modified Preferences

   Name: Value

     accessibility.typeaheadfind.flashBar: 0
     browser.cache.disk.capacity: 358400
     browser.cache.disk.smart_size_cached_value: 358400
     browser.cache.disk.smart_size.first_run: false
     browser.cache.disk.smart_size.use_old_max: false
     extensions.lastAppVersion: 38.0.1
     font.internaluseonly.changed: true
     font.name.monospace.el: Consolas
     font.name.monospace.tr: Consolas
     font.name.monospace.x-baltic: Consolas
     font.name.monospace.x-central-euro: Consolas
     font.name.monospace.x-cyrillic: Consolas
     font.name.monospace.x-unicode: Consolas
     font.name.monospace.x-western: Consolas
     font.name.sans-serif.el: Calibri
     font.name.sans-serif.tr: Calibri
     font.name.sans-serif.x-baltic: Calibri
     font.name.sans-serif.x-central-euro: Calibri
     font.name.sans-serif.x-cyrillic: Calibri
     font.name.sans-serif.x-unicode: Calibri
     font.name.sans-serif.x-western: Calibri
     font.name.serif.el: Cambria
     font.name.serif.tr: Cambria
     font.name.serif.x-baltic: Cambria
     font.name.serif.x-central-euro: Cambria
     font.name.serif.x-cyrillic: Cambria
     font.name.serif.x-unicode: Cambria
     font.name.serif.x-western: Cambria
     font.size.fixed.el: 14
     font.size.fixed.tr: 14
     font.size.fixed.x-baltic: 14
     font.size.fixed.x-central-euro: 14
     font.size.fixed.x-cyrillic: 14
     font.size.fixed.x-unicode: 14
     font.size.fixed.x-western: 14
     font.size.variable.el: 17
     font.size.variable.tr: 17
     font.size.variable.x-baltic: 17
     font.size.variable.x-central-euro: 17
     font.size.variable.x-cyrillic: 17
     font.size.variable.x-unicode: 17
     font.size.variable.x-western: 17
     mail.openMessageBehavior.version: 1
     mail.winsearch.firstRunDone: true
     mailnews.database.global.datastore.id: 06e401e7-7af5-44e9-a843-baf2e02c8f5
     mailnews.database.global.views.conversation.columns: {"threadCol":{"visible":true,"ordinal":"1"},"flaggedCol":{"visible":true,"ordinal":"3"},"attachmentCol":{"visible":false…
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1436043887
     places.history.expiration.transient_current_max_pages: 80280
     places.history.expiration.transient_optimal_database_size: 128447610
     plugin.importedState: true
     print.print_printer: \\OWNER-HP\Epson ESC/P-R
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_bgcolor: false
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_bgimages: false
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_command:
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_downloadfonts: false
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_edge_bottom: 0
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_edge_left: 0
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_edge_right: 0
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_edge_top: 0
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_evenpages: true
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_footercenter:
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_footerleft: &PT
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_footerright: &D
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_headercenter:
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_headerleft: &T
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_headerright: &U
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_in_color: true
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_margin_bottom: 0.5
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_margin_left: 0.5
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_margin_right: 0.5
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_margin_top: 0.5
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_oddpages: true
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_orientation: 1
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_page_delay: 50
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_paper_data: 1
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_paper_height: 11.00
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_paper_size_type: 0
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_paper_size_unit: 0
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_paper_width: 8.50
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_reversed: false
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_scaling: 1.00
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_shrink_to_fit: true
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_to_file: false
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_unwriteable_margin_bottom: 0
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_unwriteable_margin_left: 0
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_unwriteable_margin_right: 0
     print.printer_\\OWNER-HP\Epson_ESC/P-R.print_unwriteable_margin_top: 0
     print.printer_\\OWNER-

Troubleshooting information shows the correct version of Lightning is installed, and Lightning is enabled. Do you see the calendar at all?

What kind of calendar do you have - local or network? If network, is it a Google Calendar? If Google, how do you access it - via the Provider add-on or CalDAV?

kitten2015 said

Yesterday I downloaded the newest version of Moilla Thunderbird. System told me that my present ??? is not compatible with Lightning and to install that update. It ran forever and obviously did not install properly. Today my calendar and all my appointments are gone (it did not warn me that this might happen, so I could not save it). I tried to follow some info but nothing works - I am not able/capable to install a new calendar (does not help that I am not that good with PC's). Please help - My family relies on this calendar being updated on a daily basis. If not possible, where can I find a calendar that I can use?

Yes - I now have my calendar back - spent about an hour yesterday and re-installed Lightning and all over sudden (I have no idea what I did or did not do) my calendar was back, including all appts. I had previously entered. Thanks.