This typically happens when an antivirus program is active. Are you using one? What may have happened is that the upgrade was interrupted, causing Thunderbird to create a fresh profile. That indicates that the original profile is still on your PC, but not active. You check by looking at c:\users\<yourid>\appdata\roaming\thunderbird\profiles. There may be more than one profile folder and your addressbook and messages may be there. If so, you can reset the profile. Let us know and we'll take the next step.

Thank you very much for your help. In fact, I changed my antivirus software 3 days ago and that was probably the cause. Thank you again, you helped me a lot. Best Regards, Darek

You're welcome. Some antivirus software does offer the ability to ignore email activity in its settings.