This question is too vague to answer. Year, make, model - power locks? Key-less entry, or keyed entry or both? Quick "vague" answer - mechanic or automotive locksmith.
You should be able to find a shop manual at your local library with the right info...make copies as they are usually only in the REFERENCE section....good luck:)
First remove the Door panel on the vehicle and make sure any of the lock or handle rods are not broken or diaconected. Secondly if that's not the problem you may have a broken door latch. Price...