try pulliing the inside handle as far as possible, grab the wire with some pliers and pull hard. You probably need a new outside handle and a latch adjustment. If you can get it open, leave the door panel off until you are sure you have it right.
Open the passenger rear door, this will give you access to the latch mechanism of the front door. With a long flat screwdriver, lift the little round-ish pin protruding from the mechanism. This will release the latch and open your door. Once it's open, remove door-card and inspect the latch mechanism.
I've had this happen to me once, and just opening the door this way and closing it was ok to get the mechanism working again. Lubed all the relate components properly, and it hasn`t happened since then.
For rear doors that won't open, get in and on the back seat. Work at pulling back the trim just aft of the door opening about where the lock box is and the weatherstipping (look at the other door for approximate placement.). What you are trying to get to is latch/stricker pin that the lock box engages when the door is closed. Once you can see it (flashlight) use a flat blade screwdrive to release the mechanism and the door will open. Then you can work on it.
1991 Jetta 1.6 N/A Sold
1997 B4 TDI