Replacing the thermostat?

If all you did was change the thermostat then we need to start by checking for spark and fuel. Clearly the engine should run even if you didn't put the thermostat in it, so it has nothing to do with the engine running.

Start with spraying some starter fluid in the intake and see if it starts. If it does then we need to figure out what the cause of the fuel issue is. In not, remove the spark plug and reconnect it to the wire and how it against the block while you crank the engine and see if sparks. If not then we have an ignition issue.

https://youtu.be/dCjmRL3p4Cs

I don't think it has anything to do with the thermostat but the only thing I am thinking is may you didn't reconnect something or a fuse popped when doing it.