You didn't provide much information, so I'll guess you upgraded to a version you don't like and want to revert to a prior release. True? If so, this may work - but no guarantees. First, back up your profile to be safe. The, with Thunderbird not running, click the windows key and 'r' key (assuming you're using Windows) and on the window that opens, enter thunderbird.exe -p --allow-downgrade and press enter. This is intended to restore TB to a prior version.