Your car's seatbelts are vitally important and keep all the passengers in the car safe.
However, periodically they can get jammed or stuck. In some cases this can be a much more serious problem that needs to be repaired by the dealership.
However, you can troubleshoot this problem yourself quickly and easily. Pull on the seatbelt to try and release it from its auto-lock function.
If the seatbelt is buckled in, simply unlatch it from the cartridge and gently pull it away from the seat to release more of the belt. Then let it gradually go back into form behind the seat.
If it does not snap back into place, the belt may be tangled, twisted or there may be a jam deep within the mechanism. Pry off the plastic covers that hold the seatbelt mechanism with a screwdriver or needle nose pliers.