It sounds like the buckle is bad-the one that has the button to release the belt.
Theres a sensor and wiring in there that goes bad,pretty common on a lot of vehicles.
Also if the airbag light is on there could be a problem with the pre tensioner.Thats the belt part that is mounted to the b pillar-by the door

Try a scrapyard and see how much they want for the belts and buckles.