A jammed seatbelt jam is often caused by one of two factors:
Cleaning is the solution to dirt and grime. Retract your seatbelt until no slack remains. Next, take a vise grip or some form of a clamp and attach it to where the retractor feeds out the webbing. This will keep the seat belt from retracting back into the mechanism. Apply a spray-on cleaner (a carpet- or fabric-cleaning product is often effective) or use hot water and laundry detergent. Brush it clean. If needed, try soaking as much of the belt that you can place in a bucket of soapy water. Give it ample time to dry.
To fix a locked retractor, pull the fabric of the seatbelt as tight as you can and then give it one more yank to undo the locked belt. If this does not work, you will need to take the retractor out of your vehicle and use a screwdriver to manually spin the spool, causing the seat belt to slowly retract back into the seat belt mechanism.
When in extreme conditions, you may not have enough time to take the above actions. Prepare a seatbelt cutter with you for quick escape from the danger.