There was another user who was experiencing crashes in 8 after importing a version 7 database. Please read this thread and see if it looks like a similar issue: Version 8.1 won’t start
My advice is to export all your data from eM Client, then delete the database. Once you have restarted eM Client, add your accounts and import your data again.
If you have lots of accounts and other settings, you can export those as well just to make it a bit easier.
