if it isn't a fuse, it might be jammed up against the limit switch in the column (there is one at each end of the selt belt trac). There is an access panel in the rear seat, just as you enter the rear seat near the bottom of the seat. Behind it is a knobbed wheel that manually moves the seat belt.
If it won't move then something is binding (try jiggling the seat belt anchor in the trac). If it does move, then move it a little. It should try to move to the open position. If not, move it an inch or two and then turn the key on. It should try to move one direction or the other.
--Fred