Most jammed locks can be fixed without replacing the lock and without calling a locksmith.
Fix 1: Lubricate the Lock
Never use WD-40 inside a lock. Use a dedicated lock lubricant spray or graphite powder. Insert the nozzle into the keyway and apply a short burst. Insert the key and work it in and out several times. Turn slowly and firmly.
Fix 2: Extract a Broken Key
Do not push the broken key further in. Use needle-nose pliers to grip the exposed end and pull straight out. If no end is visible, use a broken key extractor tool.
Fix 3: Fix a Frozen Lock
Spray de-icer directly into the keyway. Alternatively heat the key with a lighter for a few seconds before inserting.
Fix 4: Fix a Misaligned Lock
Adjust the door hinges to realign the door. Alternatively chisel out the strike plate hole slightly to match the bolt position.
Fix 5: Replace the Lock Cylinder
If the key is lost or the mechanism is too worn, replace the lock cylinder. Most euro profile cylinders unscrew with one screw and pull out.