If eM Client won’t load in Windows due to a database corruption, then before redownloading all your mail etc, you can also try running the following manual database repair which sometimes will fix it depending on how bad it’s corrupted.
See @Gary post below how to manually repair eM Client for Windows.
To manually repair the database for Mac, you can also try running the following in the Terminal.
Close eM Client and then “open the Terminal app via either Launchpad or Utilities” on the Mac. Then Paste and run the following command.
/Applications/eM\ Client.app/Contents/MacOS/eM\ Client /dbrepair
Lastly if manually running the repair does not fix the database and it still fails to open, then (other than resetting up eM Client up from scratch) “if you have been making regular eM Client backups” either manually via the Menu or automatically via the Settings, then you can easily restore your most recent backup and be quickly backup and running again.
To do that, first uninstall eM Client and delete the database. Then re-download the same eM Client Pc or Mac version either via the release history page or Windows Store depending on where you originally installed it from. Then once installed “skip the setup” and restore your backup via “Menu / File / Restore”.Any new mail since the restore will update when eM Client then syncs.