First. Squirt some lock "de-icer" in there. Insert the key and work it a bit. Redo if necessary.

If that doesn't work...

Second. Squirt some "Liquid Wrench" into the mechanism. Try a couple of times.

If that doesn't work, you'll need to think of cutting the lock in some way.

Bottom line? After the de-icer and liquid wrench, your lock may be fantastic. If not, get a new lock.