When the key turns the lock won't open?

Answer 1

Lock or key may have been switched.

Lock may be rusted or burred. Oil first. Then turn key fully and have someone tap downward on the lock with a screwdriver and a hammer to unjam the lock.

Try turning the key the other way.

The key may be damaged, or the wrong one.

Borrow some large heavy duty bolt cutters, and cut the lock off. Return the bolt cutter and buy a new lock. Cutting bolts require some effort.

Good luck