What do you do if your 91 Mercury Sable overheated and now it will not run?

It depends on how hot it got. You need to get an accomplished mechanic to checkit out.

or

You can check for compresson by removing your plugs one at at time checking for compresson by letting someone turn the key while you check for compresson, if your compression are weak you might need to remove everything from the top, from intake manifolds, to head gaskets, then from the bottom you need to remove the oil pan and the muffler assembly, once the top and bottom are removed you can remove your pistons from under by pushing it upward, you will have to unbolt some nuts from the connecting rods to push out the pistons, once pistons are out check for wear and scratches on the pistons and cylinders, replace piston rings and reassemble back to how it was removed.