Download and reinstall Thunderbird. https://www.mozilla.org/en-US/thunderbird/

Thanks for your answer FredMCD That's what I am doing : I just need to rebuild my contacts and my local folders from the last backup; it means several hours of work and many problems which should result in days of work.istead of just hours It's probably due to an interruption in the automatic update.of Thunderbird Should i stop these updates?

Normally updates cause no problems. But sometimes . . .