If you have forgotten your startup password, there are some files you can delete that will give you access to the database again. None of your data or accounts will be removed, but all your settings will.
With eM Client closed open Windows Explorer and go to C:\Users\yourusername\AppData\Roaming\eM Client. You may have to enable hidden items in Explorer to get there.
Delete the three settings*.* files and restart eM Client.