Frozen lock mechanisms: If the lock mechanism itself becomes frozen, it may become difficult or impossible to turn the key in the lock. In some cases, the lock may even break if you try to force it.
Frozen key: If your key becomes frozen, it may break off in the lock or become difficult to turn. This can be especially problematic if the key breaks off in the lock, as it can be difficult to remove it without causing damage to the lock.
Expanded metal: Cold temperatures can cause metal to contract and expand. If a lock is exposed to extreme cold, the metal parts may contract, causing the lock to become loose or even fall out of the door.