lost it how? Really we know exactly what you tell us... this is a bit like posting you lost your mobile phone recently how do you get it back. More information is required to even begin to formulate a reasonable answer. What steps did you take right before this happened?

Hi, It happened after an update a few weeks ago. I tried to solve the problem myself, but no success. I hope you can help me.

Thunderbird = TB v68 series & earlier versions keep Address-Book files as *''.''mab or *''.''mab''.'' bak, etc mork based files , inside your TB-profile folder(s). [https://support.mozilla.org/en-US/questions/1317619#answer-1375337 convert mork based files into other format] (CSV,txt, etc), then you can import that into v78 series or other TB. v78 series TB uses sqlite based Address-Book files. TB uses TB-profile folders/sub-folders , those contains all emails,data,contacts,settings,etc,etc. all TB-profile sub-folders are inside/under this "''Profiles'''" folder, here: "%APPDATA%\Thunderbird\'''Profiles\'''" that is usually located here: in windows: "C:\Users\<YourWindowsUserName>\AppData\Roaming\Thunderbird\Profiles\" that ...\Thunderbird\'''Profiles'''\ folder contains multiple sub-folders , those sub-folders are various TB '''profile''' folders, a TB profile folder looks like this : "xxxxxxxx.profile-name" ''( where x in xxxxxxxx is random alphanumeric characters/letters/numbers, etc )'' ''( the "xxxxxxxx" is profile-folder-ID part , and "'''profile-name'''" is profile's name part )'' you '''MUST''' make a backup/'''copy''' of that ...\Thunderbird\'''Profiles'''\ folder in your Desktop, or where you want to. then do whatever step you prefer or appropriate. Other+Related Problems/Solutions: • [https://support.mozilla.org/en-US/questions/1316147#answer-1372552 Regain old data] : in Windows, by using downgrade process, or by using a second-TB, or by using manual file-transfer,etc). • [https://support.mozilla.org/en-US/questions/1304550#answer-1352743 Downgrade TB] in macOS: [https://support.mozilla.org/en-US/questions/1317224#answer-1374579 downgrade by using second-TB] , [https://support.mozilla.org/en-US/questions/1318142#answer-1377801 upgrade TB]. • [https://support.mozilla.org/en-US/questions/1315734#answer-1373022 ATT-Yahoo related settings for TB]. • [https://support.mozilla.org/en-US/questions/1304518 Yahoo related settings for TB]. • [https://support.mozilla.org/en-US/questions/1316618#answer-1374784 Microsoft related settings for TB]. • [https://support.mozilla.org/en-US/questions/1317619#answer-1375337 Convert mab based Address-Book into other format]. • [https://support.mozilla.org/en-US/questions/1316147#answer-1372552 Disabled auto-update of TB] : inside the linked page , click on the topside 1st link to goto the auto-update disable instruction page into another website.