Had this problem about a month ago...Most likely the issue is that the mechanism inside has frozen, probably from a little corrosion or something.
Use a coat hanger as a slim jim (unless of course you have a slim jim) to pop it open....Its pretty easy because you can roll the window down and get a pretty good look at what your doing.
Fish the hook end of the coat hanger down where the latch would be, wiggle it around until it catches on the locking mechanism, and give it a few good tugs upward. The door should pop open pretty easily. I thought it was going to be pretty hard to do, but it turned out to be a piece of cake.
Once you get the door open, squirt some oil inside (I didn't take off the inner panel, but it would be way easier if that panel is removed) and pull the handle a few times to loosen things up. I did this, and haven't had a problem in a month or two.