I am known for typing rubbish at times, and my retracting seat belt system on my mid 80`s +8 may be totally different from yours.

There is a small pendulum affair inside the belt mechanism thus the angle that MY seat belt holder relative to vertical can have an effect on both retraction and extension. if the pendulum has been triggered by being off plum AND the belt being tugged to attempt to extend it, it seems likely that the pendulum may have engaged the locking mechanism, as it is expected to do so in a collision...

There may be a chance that if you loosen the bolt holding the seat belt holder and adjust it`s angle perhaps giving the mechanism a sharp tap, or series of taps to hopefully cause the pendulum to reset and release the locking mechanism.

Best of luck in resolving your issue.