There is a pendulum/ratch device connected to the seat belt spool. Sometime when the belt is fully extended it get caught on a ratchet tooth.
If you have not already, try snapping it and relasing it a few times.
If that does not work, remove the cover by removing any screws that seem to reation it and them gently pull on it. It is reatined by plastic plugs which are generally re-useable or can be purchased at a parts store.
While holding the ratchet arem bach, start to spin the belt spool in the direction where the ratchet arm is curved like a cam as this is how the sppol is allowed to move. The other direction locks the belt, so it is not hard to figure out which way the belt goes around the spool.
Once you have a couple of inches wrapped, then pull on the belt and it should retract. If it does not and seems to have nothing to pull it on, look for the retention spring on the spool and drr if it has come out of its retaing notch.