How do you fix a door lock latch that doesn't lock?

The hook portion or the latch is probably flipped up. If you lift the exterior handle up and take a pen or reasonable facsimile, you can flip it back. Post your results.

Should someone "accidently" push down the latch, which freezes, the door won't latch or close. Pulling the outside door handle and using the pen or straight tool at the same time will release all latches!!

If the latch is flipped down, as it was on my 2000 Pontiac Firebird, then simply lifting the inner or outer latch handle may not work. My car has power door locks and I could not get the latch to release no matter how many times I cycled the lock/unlock switch. I found that I had to unlock the door with the key first, then lifting the handle worked! I don't know how the latch got "thrown" initially, but I found the seatbelt buckle stuck in the doorjamb, and I had locked the car using the keychain fob. My point is this: It will help to try and figure out what was occurring when the problem might have happened. , and Good Luck!