For a start, check whether the damaged key. It can be bent or have any other mechanical damage. If the keyof om is all right, but he hardly comes into the castle, squeeze through the keyhole a few drops of machine oil. Then insert the key and repeatedly open and close the lock.The lock may not work due to the fact that froze his movement. In this case, try to thaw the frozen lock: you can use boiling water or other means to warm up (lighter, matches, etc.) the Lock may rust if not used for a long time. Lubricate the lock mechanism, machine, or other lubricants and try to open it again.In cylindrical lock sometimes fails larva – the turning part, which is inserted into the key. It is possible to try to fix, pulled out of the lock and disassembled. Remove the plate that holds the springs and pins. Carefully remove them, remembering the exact location in the mechanism. Clean all parts and reassemble. Be careful – if you make a mistake during Assembly, the key will cease to come to the castle.In that case, if the larva is sealed, replace it with a new one. New larvae are sold at markets and in specialty stores. Take an old sample and purchase the same.Sometimes cylindrical lock stops working due to the pranks of children, pushed him in a match or other small items. Try to extract a match needle or awl. If this fails, beat the wrench with a hammer. Open the door, then try to develop the castle. The wreckage of the match gradually will be crushed, and the lock will start working. Cylindrical lock may not open if on the other hand the keyhole, inserted the key.Never lubricate the lock with oil. It gradually thickens the lock mechanism sticks and stops working.