Barrel key won't open antique cupboard lock

I have an antique 19th-century (maybe earlier) china cupboard. It locks with a barrel key that fits over a pin in the lock. I locked the cupboard door with the key, but now the key won't open the lock. The key does turn in the lock, and there's a little pressure and a clicking sound when I turn the key, but the little bolt doesn't move. How can I open this lock?