1998 Chevy Cavalier driver side door

Check the striker bolt on the door jamb, make sure it is not loose. If not, spray some penetrant in the door mechanisim. Look at the other door and note position of door mechanisim, sometimes you can use a screwdriver to pop it back into position.

It is also possible the door mechanisim is broke and would need to be replaced, try a junk yard, you should be able to pick one up for a couple bucks.