If your vehicle is showing seatbelt pre-tensioner locked up tight against the body and won't release, here are the most likely causes and the parts you'll need to fix each one.
Pulling the seatbelt all the way out engages the child-seat locking mode, which ratchets tight and won't release until fully retracted. If pulled extremely tight against an occupant, there may be no slack left to disengage the lock.
The seatbelt webbing folded over itself or twisted as it was retracting, creating a thick spot that physically jams the retractor mechanism inside the B-pillar or seat housing.
During a collision or hard impact, the vehicle fires an explosive charge in the seatbelt retractor or buckle to pull the occupant tight against the seat. Once deployed, the mechanism permanently locks and the assembly must be replaced.
The internal clock spring, pendulum, or locking gear inside the seatbelt retractor has broken or shattered, permanently jamming the belt in the locked position.