This usually means that the database is heavily corrupted so there are few options.
Some users have found that restarting Windows, or restarting eM Client a few times resolves the issue, but otherwise:
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If you have a recent backup, you can delete the current eM Client database, then restore the backup and eM Client should start working again without any problems.
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If you don’t have a backup, but all your data is synced with servers, then you can delete the database and start over. After adding your accounts, eM Client will resync with the server.
By default, the database is stored at C:\Users\yourusername\AppData\Roaming\eM Client. You may have to enable hidden items in Windows Explorer to get there. I suggest just renaming the eM Client folder, rather than deleting it, until everything is working again.