Check your firewall and see if it is blocking the new version of Thunderbird. They have a habit of doing that. The firewall should have Thunderbird set as allow or always depending on the terminology they use.

This happened before I updated TB and were the only two accounts affected. Nothing changed other than the server being upgraded from PLESK 11 to PLESK 12.

Then I would suggest that you get a current list of server settings from your provider and see if any of the settings in Thunderbird need updating.

Have you compared the settings for the account that works with the one that does not?

Yes, I checked everything, re-entered the password info on the server. One account now works once I deleted it and re-did it, the other does not work and I would like a solution that doesn't involve deleting this huge account. The admin that hosts our server does not know why the accounts stopped receiving (but not sending) as the settings did not change.

You could post your troubleshooting info and see if someone can spot a setting that needs changing.

In the Help menu is Troubleshooting Information. Click that and then select Copy Text to Clipboard

Uncheck the box to include account names

Paste that info into a reply box here.

The problem account is ACCOUNT 9 -- ACCOUNT 13 is the other account on the same server, which I fixed by deleting and adding back.

Application Basics

   Name: Thunderbird
   Version: 31.0
   User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:31.0) Gecko/20100101 Thunderbird/31.0
   Profile Folder: Show Folder

             (Local drive)
   Application Build ID: 20140717165725
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory

 Mail and News Accounts
   account2:
     INCOMING: account2, , (none) Local Folders, plain, passwordCleartext

   account6:
     INCOMING: account6, , (pop3) pop.optonline.net:110, plain, passwordCleartext
     OUTGOING: mail.optonline.net:587, plain, passwordCleartext, true

   account8:
     INCOMING: account8, , (pop3) webmail.newheadings.com:110, alwaysSTARTTLS, passwordCleartext
     OUTGOING: webmail.newheadings.com:587, alwaysSTARTTLS, passwordCleartext, true

   account9:
     INCOMING: account9, , (pop3) westportnow.com:110, alwaysSTARTTLS, passwordCleartext
     OUTGOING: mail.westportnow.com:587, alwaysSTARTTLS, passwordCleartext, true

   account11:
     INCOMING: account11, , (pop3) mail.cortinalearning.com:110, plain, passwordCleartext
     OUTGOING: mail.cortinalearning.com:2525, plain, passwordCleartext, true

   account12:
     INCOMING: account12, , (pop3) mail.leefleming.com:110, plain, passwordCleartext
     OUTGOING: mail.leefleming.com:2525, plain, passwordCleartext, true

   account13:
     INCOMING: account13, , (pop3) mail.westportnow.com:110, alwaysSTARTTLS, passwordEncrypted
     OUTGOING: mail.westportnow.com:587, alwaysSTARTTLS, passwordCleartext, true

   account14:
     INCOMING: account14, , (imap) mail.infopulsellc.com:143, plain, passwordCleartext
     OUTGOING: mail.infopulsellc.com:2525, plain, passwordCleartext, true

 Crash Reports
   http://crash-stats.mozilla.com/report/index/bp-7e87372d-7ee9-47bf-b601-38b612140710 (7/10/2014)

 Extensions
   ImportExportTools, 2.8.0.4, true, {3ed8cc52-86fc-4613-9026-c1ef969da4c3}
   Lightning, 3.3, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}
   Manually sort folders, 1.1, true, tbsortfolders@xulforum.org
   Show All Body Parts, 1.2, true, ShowAllBodyParts@kamens.us

 Important Modified Preferences

   Name: Value

     browser.cache.disk.capacity: 358400
     browser.cache.disk.smart_size.first_run: false
     browser.cache.disk.smart_size.use_old_max: false
     browser.cache.disk.smart_size_cached_value: 358400
     extensions.lastAppVersion: 31.0
     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
     gfx.direct3d.last_used_feature_level_idx: 0
     mail.openMessageBehavior.version: 1
     mail.winsearch.enable: true
     mail.winsearch.firstRunDone: true
     mail.winsearch.global_reindex_time: 1395766691
     mailnews.database.global.datastore.id: fabed0eb-3a0f-4307-a451-17228a8b57b
     network.cookie.prefsMigrated: true
     places.database.lastMaintenance: 1406337469
     places.history.expiration.transient_current_max_pages: 103611
     plugin.importedState: true

 Graphics

     Adapter Description: NVIDIA NVS 2100M
     Vendor ID: 0x10de
     Device ID: 0x0a6a
     Adapter RAM: 512
     Adapter Drivers: nvd3dumx,nvwgf2umx,nvwgf2umx nvd3dum,nvwgf2um,nvwgf2um
     Driver Version: 9.18.13.2702
     Driver Date: 8-29-2013
     Direct2D Enabled: true
     DirectWrite Enabled: true (6.2.9200.16571)
     ClearType Parameters: Gamma: 2200 Pixel Structure: R ClearType Level: 100 Enhanced Contrast: 100
     WebGL Renderer: false
     GPU Accelerated Windows: 2/2 Direct3D 10

     AzureCanvasBackend: direct2d
     AzureSkiaAccelerated: 0
     AzureFallbackCanvasBackend: cairo
     AzureContentBackend: direct2d

 JavaScript

 Incremental GC: 1

 Accessibility

   Activated: 0
   Prevent Accessibility: 0

 Library Versions

     Expected minimum version
     Version in use

     NSPR
     4.10.6
     4.10.6

     NSS
     3.16.2 Basic ECC
     3.16.2 Basic ECC

     NSS Util
     3.16.2
     3.16.2

     NSS SSL
     3.16.2 Basic ECC
     3.16.2 Basic ECC

     NSS S/MIME
     3.16.2 Basic ECC
     3.16.2 Basic ECC

Account 9 - westportnow.com:110

Account 13 - mail.westportnow.com:110

Different server names.

THANK YOU!