Overheated while driving now it won't start

UGH, not good.

How long were you driving before you noticed the missing cap, it must have been overheating?????

Probably some nasty damage to the engine.

You can do a very simple evaluation for engine damage. Disconnect battery then put a socket and ratchet on the crank-bolt and turn the ratchet clockwise......it should move with some effort and eventually get harder to turn as the valve close. If it does not turn or turns very easily, this is an indicator of something wrong with the engine.

We replaced an engine in a chevy astrovan about ten years ago. I checked her water and there wasn't any at all in the engine. I asked her "was it leaking" and she said "yes but it stopped". Obviously it stopped. It was out of water. In her case the block was cracked requiring an engine replacement. It's possible it may start when you add water. If you don't need to add maybe two gallons, then it was merely low. The reason head gaskets blow in this situation is they are softened in heat and lose their ability to hold back compression so if you overheat the engine they can let loose in many different ways. You could get exhaust into the radiator or coolant out of the exhaust. that MAY be all you did if you even did that, because the bottom of the engine is kept cool for a while with oil. If that gets overheated, you'll no longer be driving the car because the engine seizes when it's overheated to the point the oil is burning dry. I have experienced this and so has my wife. I was driving an 82 Caddy and the check engine light was in the center of the dash where sunlight could hit it so i didn't see it on. Bottom radiator hose went. My clue was the car slowing down. When it cooled off i was able to drive it, but the heads were destroyed. It would blow all of the water out of the radiator within a mile.