Unfortunately there is no way to reset or retrieve the password, but you can delete your settings file which will remove the startup password. It will also remove some of your settings, but not your data, account settings, or signatures etc.
To do that, with eM Client closed, go to C:\Users\yourusername\AppData\Roaming\eM Client\ in Windows File Explorer. You may have to enable hidden items to get there. Delete any settings*.* files and restart eM Client.
If you have the database in another location, then it will be that one, rather than C:\Users\yourusername\AppData\Roaming\eM Client\ .