1992 Ford Taurus will not start

Start out by double checking spark and fuel. Make sure you r distributor is turning. Take off 1 wire, and while cranking, you should get a pulsing spark. Next, verify fuel pressure at the test port on the fuel rail. Best to use a gauge, and check the pressure.
Next, check the electrical connectors in the area where you were working to replace the thermostat. You have a connector for the fuel injectors near the thermostat that powers the injectors. Make sure that all connectors are fully plugged in and latched.
Most likely, one of the connectors was disturbed while you were working near it, and may have come partially unplugged.