How do I remove broken thermostat housing bolts?

What is your question?

I was trying to replace the thermostat and had removed the alternator and was able to get to the bolts but the first one just snapped off when I tried to remove it. I had to replace everything due to time constraints and need to repair it soon because it is leaking water badly. I have the radiator cap on but not locked down to keep the pressure from blowing out all of the water in the system. Any help would be appreciated.