[user104658]

Honestly... if you don't know, then you probably need someone else to do it. I'm all for people doing things themselves - but really if you don't even know if the hinge is just loose or broken, you'd be better getting someone who can do it to come round, and watch them/ask questions so you'll know next time.

As a total guess, for the one that's come off entirely, if the hinge itself isn't broken then the screw holes have probably degraded (widened) so you won't be able to screw it back on in the right place (for it to sit straight and close properly) without plugging the holes a bit.