My wife was driving her 2007 Explorer V-6 (4.2L, I think) last night and it overheated. She was close to home and tried to drive it on in, but didn't make it. The engine stalled and the car had to be towed home. Now, after cooling off all night, and with new coolant added, it won't start.
Being mostly ignorant about electrical systems built after 1974, I disconnected and reconnected the battery, in case the computer needed a restart. I also checked all the fuses, just in case.
The engine turns over, but won't start. It seems to turn over a bit slow, but I'm not really sure about that, since I practically never drive the vehicle. I used a jump-start to boost the battery, but it made no difference.
What I'm most interested in knowing, at this point, is whether the engine is ruined. If anyone can make an educated guess on this scant information, I would appreciate it. Or, I might be able to answer more questions.
If you happen to have a compression tester you could check the compression on a few cylinders. It might not hurt to remove all spark plugs, drain the oil and refill with fresh oil, use the starter to spin the engine over a few times with the fresh oil to get it worked in to the bearings, etc., then reinstall plugs and see if it will fire up.
Senix and shorod pretty much have it covered, but I'll add a little to it. Overheated engine + aluminum heads = not good, based on my bad experience with a supposedly good Honda. Even just getting to the upper end of the "normal" range is not good.
If you pull the plugs, look for signs of water on them. When my Honda didn't start and I pulled the plugs, one was dripping with coolant. Just this one cylinder not firing was enough to prevent the engine from starting.
A compression test as shorod suggests will pick up a bad head (cracked, warped or burned valves) or worn cylinder. That may be all you need, but, if you want to take it further, a leakdown test will tell you where the air is leaking - past the valves and out the intake or exhaust, past the piston rings (out the crankcase vent), or into the water jacket and eventually into the radiator. You don't need the fancy tool, just a spark plug adapter and compressed air source.
Draining the oil will tell you if there's coolant in the oil. Coolant in the oil does bad things to the bearings unless the problem was discovered and corrected quickly. You should also look for the presence of metallic debris in the oil.