I would do things in this order to try and free it up. (1) holding the lock so that the keyhole is pointing downwards, tap it with a hammer to allow any foreign items to come out of the lock, I would do this in different directions to dislodge anything in the keyway. (2) If you have access to compressed air I would blast air into the keyway to remove anything lodged inside. (3) Use some lock lubricant inside the lock and repeat step 1 with plenty of lubricant. (4) Gently insert the key and slowly and gently jiggle the key inside the lock, removing it regularly and tapping the lock again before inserting it. Finally, if all this does not work after trying several times I would simply cut the lock and get a new one.