Repeated insertion of a key into a front door lock can cause dirt and debris to build up in the key slot. This results in the key being unable to open the lock. Clean out the front door lock with some common household supplies. A locksmith is not needed, nor must you purchase any special equipment. The procedure is straightforward, will not damage the lock in any way and will leave the lock functioning properly.
Apply light machine oil to an edge of a small, thin, disposable lint-free cloth. Place the edge of the cloth into the slot of the front door lock. Push the cloth as far into the slot as possible, using the key that locks and unlocks the front door.
Push the key in and out of the lock. Remove the cloth. Repeat this procedure with another clean edge of the cloth, after first having applied light machine oil to it.
Apply light machine oil to the serrated edges of the key. Insert the key in the front door lock. Turn the key to lock and then unlock the door. Repeat this procedure two or three more times until the locking mechanism is responding without sticking in any way. Wipe the machine oil off the key with the cloth. Dispose of the cloth in the trash.
Forcing the key in the front door lock can break the key.