These locks are usually mounted above the door knob lock. They might be called night latches, rim bolts, slam locks, jimmyproof locks or surface mount locks.
They usually have a key cylinder on the outside of the door and a large body on the inside of the door that will lock or latch into a surface strike mounted on the door frame.
What you can do - Remove the lock and have the cylinder rekeyed or replaced.
You must remove the inside lock body. It will be held by screws that might be standard screws or one-way security screws. One-way screws can be hard to remove. If you have a dremel tool(trademark) you can cut a slot in the head of the screw. Or you can use an old chisel to tap on the edge of the screw to drive it counter clockwise until you back it out enough to grab it with a pliers and remove it.
If you don't see any screws, they may be hidden. There may be a plate that has to turn or come off. You may have to turn the key to remove the plate.
When you have removed the lock body, you will see the plate and screws that are holding the outside cylinder to the door.
Remove the cylinder and take it to a lock shop for an exact replacement or have it rekeyed. If your lock uses a key on the inside, take that part to be changed too. Bring a working key, if you have one.
Finally, if you buy a replacement cylinder, you will probably have to cut the tail-piece and cut the screws to the correct length. Then reinstall. Some locks have a shutter that may be blocking the tail-piece from going back in. Locate the spring and gently press on the spring while pushing the lever to open the shutter.
What a locksmith can do - Rekey your lock and possibly match your other house key so you have less keys to carry.
These locks are probably the easiest for a do-it-yourselfer to install. That is probably why they can be a bear to reinstall. The original installation, if not done correctly, can make it difficult to get things to line up and work easily when you reinstall.
An experienced locksmith may have to shim here and there or re-drill the holes before he can reinstall the lock he took off and rekeyed.