Funny, someone else had the problem recently.
Hide / unhide works by levels.
If you enter a group, hide a face, then exit the group, you can’t unhide the face. because you’re outside of the group. you have to go back inside the group, THEN unhide.
This is why I tend not to hide raw geometry, at least groups and components are in the outliner.
In the View menu, you can activate “hidden objects”, that’ll show you in some sort of translucent grid what is hidden. It might help you select it and unhide it
edit : if it doesn’t do the trick, please, share a problematic file with us here (drag it in the text window, or through wetransfer) so we can see in detail where it’s causing troubles