Seat belt not retracting

I've run into that sort of situation a couple of times now. In the first case it was because the belt got twisted while retracting and the extra bulk caused by the twist jammed up the mechanism so it didn't work any more. In that case I ended up taking the trim panel over the seat belt mechanism off and managed to work the belt out far enough to free the twist. After that, it was just a matter of letting it retract normally, then replacing the trim and it was fine.

In the second case I encountered I wasn't so lucky. A piece inside the belt mechanism had broken and I ended up replacing the entire seat belt.

Hopefully it's just a twisted belt, but in ether case you'll need to pull the trim panel of the seat belt along the pillar and examine the mechanism to figure out what's wrong. If it's a twist it'll be obvious.