The main burner will not light! The pilot stays on fine but when the thermostat is raised, the main burner won't light. I've checked the electrical box, everything is fine. I even changed the thermostat but it still will not ignite the main burner. Help, I'm freezing!

Well you will never get an accurate answer based solely upon the info you provided.
Furnace type?
Model #?
Serial #?

With a volt meter check the voltage at the top of the main gas valve, shile the thermostat is calling for heat, it should be 24 volts.

Sorry, first timer.

Trane Gas Fired Forced Air
Mod#: BLU108E93681
Ser#: S42599089