help! trunk won't open...

[desertfox]

I'm relatively new to my '79 240d, so I've only had a few occasions to actually work on the car, but it's thrown a real curve ball at me this time. I went to open my trunk and the key fits in the lock but turns as though it's not catching anything, I can move it from right to left and back again, but nothing happens. I suppose I could call a locksmith, but I am wondering if anyone has any suggestions for how to open a lock that's not jammed, but seemingly too free...
Thanks
Josh