If it is master keyed then it is not likely a lock/door that you own. Most homeowners don't have a need for a master key system. I'm guessing that you live in an apartment or maybe a dorm or just a rent house.
If that's the case you need to contact your landlord or whoever oversees that property so they can have it properly rekeyed to the master as well as your key.
If it is indeed your own property then do as stated above and replace the lock or call as locksmith and ignore what I just said.
