p0125 on a ford escort

OK I own a ford escort Zx2 year 2000
I was driving home the other day and the S.E.S went on I went to a auto store and found out that it was a p0125 and that it might be the thermostate but I got up today to go to the auto store and get me a new thermostate but my engine wont turn over I cant figure it out my engine never overheated and I have no leaks that I can see so why wont the dang thing start?

so my Question is can a open valve really make it so your car wont turn over (I have never heard of such a thing if it can) or is their another problem that I'm not adressing?

I almost forgot when I try to turn the ignition it almost sounds like I have a pluged fuel filter if that helps anyone?

did the coolant mix with the engine oil? maybe the engine is hydro locked!
Will it turn over with a boost? Do the headlights go dim when it's cranked?
can your hear the starter clicking? A code is probably not you biggest problem at this point! I'd get it towed to a mechanic at this point!