NEVER open the cap on a hot cooling system. You're lucky you weren't seriously burned.
When my car overheated, it turned out to be a gummed up pisser hose -- that's the one that leads from the radiator to the expansion tank. I loosened that hose, cleared it out, and all was fine. Your mileage may vary. There are lots of things that can cause overheating.
You've probably not done any serious damage to your car. Time will tell.