Quite possibly the engine got so hot that it was what is called "dieseling" heat of the engine was enough to fire the fuel in the cylinders.
Now as to why it won't start. Hopefully you didn't blow a head gasket or worse.
Pull the dipstick and look for water in the oil. If you see a milkshake looking oil, forget trying to start it, call a tow truck.
If the oil looks good, pull a plug wire off and have a victim, err helper hold it close to the engine block while you try and start the engine. If he gets zapped, or sees a spark arcing, you got spark.
If not, take the distributor cap off and check for moisture from the leaking rad. If it's got moisture in it the spark will arc inside and not be enough getting to the plugs.
Dry it out best as you can, and shoot some WD-40 in the cap to help clear out any remaining moisture.
Also could be the Crankshaft Position Sensor, CPS, got wet and fried. If it did, you will get no spark.
Let us know what you find.