you may have to remove the bolt that holds the buckle to the floor. Try to push the belt into the retractor and see if it frees up.
I assume you are saying it is in the locked mode. It has an automatic lock mode for holding a child seat. When the belt is pulled all the way out, it goes into locking mode. You will hear it ratchet when the belt starts going back in.
Section 01-20A: Restraints, Active
1999 Taurus, Sable Workshop Manual
DESCRIPTION AND OPERATION
Safety Belt, Lap/Shoulder
The dual locking mode retractor on the shoulder belt portion of the combination lap/shoulder safety belt for the front seat passenger and rear seat outboard passengers operates in two ways:
In the vehicle sensitive (emergency locking) mode, the shoulder belt retractor will allow the occupant freedom of movement, locking tight only on hard braking, hard cornering or impacts of approximately 8 km/h (5 mph) or more. The front and rear outboard seat belt retractors can also be made to lock by pulling/jerking on the belt.
In the automatic locking mode, the shoulder belt retractor will be automatically locked and remain locked when the combination lap/shoulder safety belt is buckled and does not allow the occupant freedom of movement. This mode provides tight lap/shoulder belt fit on the occupant and on a child safety seat or infant carrier installation restraint.
WARNING: REAR-FACING INFANT SEATS SHOULD NEVER BE PLACED IN THE FRONT SEATS.
When the combination lap/shoulder belt is unbuckled and allowed to retract completely, the retractor will switch to the vehicle sensitive (emergency) locking mode.
The automatic locking mode must be used when installing a child safety seat on the front passenger seat and rear outboard seats where dual locking retractors are provided