If you can't open it, probably a one of the rods that connect to the handle busted.
If the tailgate is stuck closed, then it is a PITA to get in there and work on the internals. You'll need to clean out the back of the truck and pop off the access panel from the inside. Make sure you bring adequate lighting.
The good news is I recently gutted the tailgate of my parts truck, so whatever you did break, I probably have a replacement for you. :thumbup