Seat Belt Lockout on 300E

Car: 1987 300E with SRS.

I had a driveshaft coupling disintegrate last week. For some unknown reason, the airbag deployed when the joint broke. Anyway, on receiving the repaired car back from the shop with a new coupling and airbag in place, the driver's side seat belt is tensioned tight and locked up and the SRS warning light is illuminated. How do I re-set it? I'm told that I have to replace the seat belt retractor, surely not? How did a drive shaft joint failure pop the airbag and lock up the seat belt? The incident occurred at low speed without a rapid stop or collision. Anybody help?

Mike B