Bad Thermostat most likely, now check engine...?

So three days ago, in the morning after the engine cooled all night, I was driving to work when my temp gauge went past the normal halfway point and continued to climb into the red. At that point, no heat was coming out, just cold air. I checked my coolant level and realized I was low, so I filled it with water for the time being. For the next 20 miles or so, the gauge was at a normal level. I got coolant, put it in, when I drove the next day, my temp gauge climbed back up to the red and only cool air came out.
I was told since it wasn't the coolant level, it's most likely the thermostat. I haven't had the time to replace this and on drive to work today, temp gauge up all the way, engine smoking, and finally a check engine light and little acceleration.
Will it still just be a simple thermostat replacement, or will there most likely be additional problems to fix?

it sounds like you ran low on coolant(need to find the leak)when you filled the
system you may need to bleed off any air thats trapped in system.
as far as thermo stuck ???it may be but i would'nt bank on it
the year & engine would help!!!since your in the honda forum i guess its a honda

It could be a more complicated issue. There could be a problem with the radiator or the water pump. It's best to get this checked out by a mechanic ASAP.

Sounds like you may have over heated it one too many times. I wouldn't waste money one t/stat until some one verifies it does not have a blown head gasket.