Coolant frozen?

[Pimpin Dsmstyle]

I never park outside. Well Saturday night I parked outside because I was tired and lazy. I came out on Sunday morning, started the car, and made my way inside the house. I walked out 10 minutes later and my car was smoking like hell. I popped the hood and noticed that my overflow tank was just boiling and what not. My temp was high, but not over heated yet.

I pulled it into the garage, let it sit for awhile. I came back a few hours later and poured 50/50 HOT water and antifreeze in the cooling system. It didn't seem to smoke ( I let it get to temp, and the only weird thing was that it was idling at 2k.) Could it have been that my either my coolant was frozen from having too much water compaired to antifreeze? Could it have been my thermostat was frozen shut? Ideas? I do not want to drive it until I know this problem is fixed.