Hi Michael, I’m afraid this means your database is corrupt, this may be due to a improper initialization of the application or your computer. Do you have a backup of your data? Are you using an IMAP account(s)?
At this point you’ll have to setup a new database, to do so, please go to your local folder > App Data > Roaming, and rename or remove your eM Client folder. After you open the application again, it will create a new database in this folder, allowing you to setup your account again.
Hope this helps,
Paul.