Since it cranks, it's not an problem with battery, alternator, etc. It is more likely a fuel problem such as a plugged fuel filter or a bad fuel pump/injector.
Starting problems are usually a sign of a bad starter. If there is cranking when the key is the RUN position and you hear grinding or clicking, replace the starter.
The StopLeak and thermostat should not affect he engine's ability to start. Check to make sure that you are getting a good spark fro mall the plugs and there is fuel getting to them.
First make sure that the battery is not completely dead. Use a jump starter and see if that make's a difference. If it doesn't, make sure that there is gas in it. If this does not rectify the problem it could be the fuel or water pump.
