The locking mechanism inside the doorknob of a house door is prone to failure due to the constant insertion and removal of the occupant’s house key. You can fix the locking mechanism inside of a doorknob that has jammed due to a house key having broken off inside of it. You will need to prepare a hardware tool and have some supplies from a hardware store to fix the lock so that it will once again work as it should. No locksmithing abilities are required.
Buy a hobby-style hacksaw blade from a hardware store, building supply store or hobby shop. Put on the work gloves. Put on the protective eye gear.
Hold the hacksaw blade with the teeth pointing down. Insert the harpoon-like end of the hacksaw blade into the doorknob’s locking mechanism so the teeth of the hacksaw blade are above the nubs cut into the key.
Smooth the replacment house key’s tip gently to remove burrs with the nail file.