Try disabling Full-screen View (Ctrl + Shift + F)
If that does not work your digikam probably got stuck in the fullscreen mode.
I could not fix that in Mint Cinnamon (there is a bug report on that) so I actually had to distrohop to openSUSE KDE.
Sometimes you can fix that via modifying the digikamrc config file (I think it is in your home directory in .config which is a hidden folder.