If key doesn't turn and theres no external sign of damage key / tumblers may be worn, you can remove the barrel and file the tumblers. If it does turn the rod inthe dor may have come off or need bending / adjusting. IF theres any sign of extrnal damage you've probably had a chav stick a screwdriver it in.
If you're talking central locking the slidign door contact is made out of rubbish and tprobably rusted away onthe door side, its at the bottom rear side of the door near the rail.