Why is my deadbolt won't turn and how can I fix it?

A deadbolt that won't turn may be due to misalignment, a worn-out lock, or a stuck mechanism. To fix it, try lubricating the lock with graphite powder or WD-40, check for any obstructions in the lock mechanism, and adjust the strike plate if needed. If these steps don't work, consider calling a locksmith for further assistance.