The seat likely has a manual lever or crank somewhere on the side of the seat, usually near the seat bottom or the seat back itself. Look for a lever or a rotating knob. If it's a manual adjustment, you'll need to use that lever or knob to move the seat back.
If you can't find a manual lever, it's *possible* (though less likely in an '86 Regal) the seat has some form of power adjustment. If this is the case, you'll need to check the fuse box to ensure the relevant fuse isn't blown, and even then, the power seat mechanism should operate independently of the engine being inoperative.
If you can't find any controls, it's possible the seat adjustment mechanism is broken or jammed. You might need to investigate the seat itself to try and figure out what's preventing it from moving. You could check under the seat for any obstructions or broken parts.
