The latching/locking mechanism is a bunch of steel rods inside the door that couple all the bits together. At a few points they're "hinged" by plastic bits. One of the plastic bits has broken.
When it happened to me, I was lucky that I could open it from the outside. You need to get the door open in order to be able to remove the inside side panel in order to find & replace the broken bit. Most likely what's happened is the lock itself is stuck. Your best bet is to take it to a dealer and have them try to open it, perhaps using a slimjim or something.
Thank you Stepho for the advice. I was able to get the door open by just using the handle a few hundred times. I could see the mechanism working and all the rods were moving and attached. The problem was that the door latch mechanism was rusty and not releasing all the way. After taking the door panel off and spraying the whole latch assembly with DW40, it works perfectly!!
cameron61992 , I rolled the window down so I could see the mechanism working to make sure that nothing was unattached or broken so I could evaluate the problem. Once I was able to tell that it was all working (from observing the other door that would open), I knew it was something with the latch mechanism. I just got lucky with it finally opening but the solution was to spray it with DW 40. You might try spraying the mechanism with DW 40 to help free any rust that is not allowing the latch to move in fully. If any of the rods that go to the door handle or lock are not moving, then you might have a different problem. Hope this helps.