Driver seatbelt retractor won't release

I am not sure I understand the problem, do you mean that the button will not release the seat belt or that the belt is stuck and you can not pull it from the "b" pillar.
If you cannot pull the belt from the "b" pillar, remove the anchor bolt from the seat and allow the belt to retract slightly and then pull it out and reattach the anchor, after that, it should work normally. If the warranty is still in effect, have BMW replace the defective unit. If not, purchase a replacement unit and install it or have it replaced.