Why your seat belt locked up and will not move
A modern seat belt has a small pyrotechnic charge built into the assembly called a pretensioner. The instant the airbag control module reads a crash, it sends current to that charge, the charge fires, and a piston or rotary mechanism yanks the slack out of the belt and pulls it tight against your body in a few thousandths of a second. That is what holds you in the seat before the airbag ever inflates.
The catch is that the charge is a one-time device. A pretensioner fires once in a deployment and then it is spent. It cannot reset itself, and there is no button to re-arm it. After the charge goes off, the retractor usually locks solid, the buckle latch may seize, and the webbing can be scorched or frayed where it was pulled hard through the guide. The belt is now dead weight. It will not pull out, it will not retract, and it will not protect anyone in another collision.
A dealer answers this one way: throw the whole assembly in the trash and sell you a brand-new one, often for several hundred dollars per belt. We answer it a different way. We take your original belt apart in our shop, install a fresh pretensioner charge, free the locked retractor, repair or replace the buckle, and put new webbing on if yours is damaged. When we are done, your belt holds to factory specification again, because it is the same belt the engineers built the car around. A properly repaired belt is every bit as strong as the one that came off the line.