There is a coil spring inside the mechanism that is under tention when pulled out and forces itself to relax when trying to retract.
Unbolt the seat belt assembly from the floorboard and B-pillar, (have to unplug seat belt warning sensor also I think) put the entire thing on a work bench. Pull the plastic over up off the mechanism and out of the way enough to see everything. Everything under the cover should be clean, not broken and the spring should be wound up and holding it's position. If it's dirty or has debris in it, if the spring is broken or another mechanism is broken or not attached, reattach it and/or replace it as needed. Be careful with the spring as it can be a bear to wind back up and if it slips as you are trying to install it, it will go bouncing around like a kangaroo.
If you really want too, you can buy a brand new seat belt assembly, but it's not hard to fix the old ones usually. Took me about 30 min to fix the one in my 73.