The deadlock that stopped being used
A back door has a good deadlock that has not been thrown in two years. The door has dropped slightly, so it must be lifted and held against the frame while the key turns, and that has become more than the householder can manage.
This reads as a security lapse and is actually a carpentry problem. Rehanging or adjusting the door so the bolt meets its keeper without force restores the lock to use immediately. Nothing needed replacing, and a door that had effectively been latch-only for two years is properly locked again.