If you have an IMAP account, you can recreate the database fairly easily.
1. Close eM Client
2. Change the name or delete the following directory (or copy it to another location and delete the original):
c:\users[user_name]\Appdata\Roaming\eM Client
this is a hidden directory, so make sure you have “Hidden Items” checked in Windows file browser.
3. Restart em Client, which will create a new, blank database.
4. reenter your account information and the database will re-sync itself.
If your are using POP3, it is a bit more problematic, as emails are generally not left on the server and unless you have backed up your account, there is not much you can do.
The sad fact is all databases are subject to corruption. eM Client has a backup utility that can be scheduled to run in the background. Best to use it.