Database Corruption - clementine-player/Clementine GitHub Wiki
The page is WIP.
Currently, the only suggestion in case of DB problems is completely deleting the old DB. It's located in your home folder in ~/.config
So, if you exit Clementine and run the commands below, it'll rebuild the database and hopefully everything will be alright.
rm ~/.config/Clementine/clementine.db
rm ~/.config/Clementine/clementine.db.bak
rm ~/.config/Clementine/jamendo.db