Try seeing if you can move the seat belt in further. Mine used to do this when you pulled the seat belt all the way out, then let it back in. This may be what happened to you, and you just need to push the seat belt in further. If not, there is probably just a connector needing unplugging for the belt locking.
I went out to my car, and tried pulling on the seat belts and hard and fast as I could, and no matter what I did, I couldn't get them to lock. I don't have a battery in my car, so nothing electronic works, so I can pretty much guaruntee if I went out there, put in a battery, and repeated the same process, I could get them to lock.
'89 SSE - GX3 -190k, 155k on body - Back for another ride.