2002 nissan xterra l/f seat belt locked up

Try spraying penetrating lubricant or graphite into it, wiggle and try. Spray inside and press the button and spray between the button and the housing. If this dosen't work you can try looking into the seat belt with a flash light and see if you can see anything moving compare with a belt that works properly. If not successful you will have to replace the seat belt assembly as they cannot be taken apart as they are a safety item and are replaced as an assembly.