A seat belt can get stuck in its latch for a number of reasons such as like age, rust or weather. Both the seat belt and its latch are made of metal connected to plastic casings, and breaking the casings to free the seat belt would cause a need for a costly repair. Work to ease the seat belt free of where it's stuck to keep the mechanism whole and regain use of the seat belt.
Put your seat in its fully upright position.
Push the seat belt into the latch so that it is fully caught in the mechanism. Be gentle.
Press the nozzle of the WD-40 can against the lips of the seat belt mechanism. Spray two quick bursts into the lips so that the liquid runs down over the seat belt and latch.
Jiggle the seat belt gently in the mechanism. Wait three minutes, then press the unlock button on the seat belt latch and gently pull the seat belt from the mechanism.
Wiggle the seat belt back and forth slowly to ease it out of the mechanism if it doesn't slide right out or is stuck in an uneven position. Apple more WD-40, and repeat the steps if it doesn't come free immediately.