You may be able to get it unlocked by doing what mgertsch says. But if you do get it unlocked, you will probably experience oil burning and/or knocking and/or low compression and the engine won't last much longer. To repair it you would have to remove the engine and completely rebuild it purchasing all new gaskets, bearings, maybe cam/lifter, and piston rings. You will also have to have machine work done. It is hard to say without pulling it apart.
If it turns out that it is locked up, I would just buy a used low mileage engine from a junkyard and swap it in. It will be cheaper more than likely and definitely less time consuming.
If it just over heated and quit then the motor isn't blown. If it over heated and went poof and oil and parts went everywhere then its blown...
I would drain the fluids and inspect them for contamination of oil or antifreeze in either or. You could just put a breaker bar on the crank and roll it over. Drop the pan and see if there is any visible damage if you don't feel anything rolling it over or see and damage I would fill it back up and crank it over and see if it starts...Hell you mights just have a bad oil pump. Youcan run it for a little bit before it heats up to casue damage and by then you can usually figure it out...
OUT WITH THE OLD R.I.P. 98 TJ Sport 4" Procomp lift/adjustable lower control arms and track bar **** Cepek FCII 33x12.50x15/**** Cepek DC-1 wheels
IN WITH THE NEW 05 WK LIMITED EDITION 5.7L
If the engine got hot enuff to seize up there could any number of problems. Cracked or warped head, If it hydro-locked (from liquid in the cylinders) it could have broken rings, bent or broken connecting rods. I'd check that engine very close before i put any money in it. It could be fine and dandy, just fix why it overheated in the first place and drive on. But you never can tell without a good inspection.
79 CJ 5, fiberglass tub on a cut down full size chevy frame. Full width axles GM 14 bolt in the rear. Dana 60 up front both with 5.38 gears. 350/400 turbo, np205 transfer case. Custom built modified 3 link rear coil spring suspension in the back, skyjacker leafs in the front. 44X18.5 gumbo mudders. 104" wheelbase hood and doghouse stretched 13" . ramsey 8000lb winch.