Moisture gets into cylinders, temperatures drop, and suddenly your key will not turn on a January morning. Prevention is simple: keep locks lubricated with a proper lock lubricant (graphite or PTFE — never oil, which gums up in the cold), and make sure exterior doors and gates are adjusted so they latch without force.
If a lock is already frozen, warm the key — not the lock — with your hands or warm (not boiling) water, then work it gently. De-icer products made for locks help; a lighter on the key is a classic for a reason. Never force a key in a frozen lock; snapped keys in January are a miserable emergency.
The deeper fix is seasonal maintenance: we clean and lubricate cylinders, adjust strikes and closers that shifted with temperature swings, and replace weather-exposed hardware before it fails. One autumn visit prevents most winter lock emergencies.