Join Date: Aug 2016
Location: West Coast Central Scotland
If you are under warranty, put it in for repair.
Assume you are out of warranty and have to sort it yourself.
If you can get behind the trim, you may find the seat belt has twisted itself and hence jambed up, which is just a easy untwist to fix. (I had that on a previous Vauxhall car, and it was an easy fix).
If not then it is a very sensitive belt to locking up easily by pulling hard* and requires Freddys method as mentioned above. Other than that it will be as Freddy says, to take it to a garage and likely replacing it at cost.
( *The locking up by a fast jerk is necessary for crash purposes, but some belts are far more sensitive than others, and can be a nuance to put on and retract).