If you upgraded from eM Client 6, your data hasn't been actually lost - you just accidentally skipped database migration.
Since eM Client 7, we have created a completely new database structure for more efficient data storage, so the data cannot be just loaded from the old database as was possible in the past.

To migrate your data manually please use the Menu > File > Import.. > Import from eM Client 6 option.

If you upgraded from eM Client 7, the database should be upgraded automatically, since eM Client 8 keeps to the same model and can upgrade these files automatically, no migration is needed.
In this case, the issue might be with your storage location - make sure that the storage folder in Menu > Settings > General > Storage is pointing to the correct location where your eM Client 7 database was stored.