TROUBLED AND LOST BLOWING COOLANT

I am a proud owner of a 95 celica except for right now its blowing coolant from the overlow and i dont know why i am lost yesterday my boyfriend changed the thermastat and the car ran great for most the day then all of a suddend it started overheating and blowing steam. we were puzzled at why. We took the car and parked it in the garage well he put the old thermastat back on and now i have no heat and its blowing coolant from the overflow but not over heating and im lost we did have to replace the radiator cap and patch a small hole in the top of the raidator on the top plastic part idk and im lost