I'm actually having that problem right now too! it worked fine earlier tonight, but I tried to open my trunk after some christmas shopping to put them inside and nothing.. my remote, the switch, the tumbler.. my key goes into the lock, but wont turn. so Im gonna have to use some wd40 (or something similar) to free it up so i can turn it.
so for you id say get some wd40 and spray inside the lock try and free it up, use some pliers to hold the key and turn it.. but dont break the key in the lock.. bad move.. bad!! if you cant free it you will have to drill it out so you can get in, but you will have to replace the tumbler in the lock assembly (not sure if its a tumbler itself of the entire latch assembly).
as for the cause of the problem, it may be broken wires in the trunk hinge. Im thinkin that is what the problem is with mine, but have to get my trunk open to check it out!