In my stone age days as a mechanic I would have just given the housing a tap with a hammer as the old belts had a large ball bearing and a tilting plate so that when you braked or hit something the ball bearing would roll and tilt the plate thus locking the belt.
Now I believe the belts have electronic sensors in the housing so DONT hit it with a hammer. If it is still attached to the housing, make sure the car is on the level and try gently pulling and releasing the tension on the belt. It might release. Don't yank it.
If all else fails your local friendly garage might be able to help.
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).