The key may be damaged or worn and no longer fits properly in the cylinder.
Thumb turn lock jammed.
If there s a thumb turn try actuating the bolt with it.
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The solutions and severity of this issue will differ depending on what type of lock it is.
The most pressing will be when your deadbolt door lock is stuck in the open position because your deadbolt provides the real security.
After enough use you might find that your door lock has begun to stick making it difficult to put in turn or pull out your key.
A jammed lock can cause someone to miss work be late for appointments and in general wreak havoc in a daily routine.
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If the lock is old the bolt or the intervals may be gummed up or rusted and seized up in the open position.
With your door lock stuck in unlocked the position your home can be made vulnerable.
Door lock stuck in open position.
The last resort is to disassemble the lock to see if something has jammed or is broken you may be able to set it straight or replace the part without buying a whole new lock.
How to fix a sticky lock.