Hello, There is a 1/4" diameter aluminum tube running off the main gas valve. follow that tube.. at the end of it is the pilot light.
Turn the gas valve knob to 'pilot'.. depress at knob or button, hold it down while you use a long match or barbeque pit lighter to lite the pilot... KEEP HOLDING THE BUTTON DOWN for at least 60 seconds. when you let up on the button the pilot should stay lit
THEN carefully rotate the knob to ON,... be sure you do not pass the OFF position but go directly to ON... this will put gas to the burners if your thermostat is calling for heat.
Please click the 'accept' button if this helped, Phil
Hmmm. That should have lit the main burner. And you say the thermostat is ON as usual... something else is killing power to the gas valve in that case. are you sure you rotated the gas valve knob back to the ON position? check that, its easy to misread the pointer.
Do you have an electrical tester? My guess is no... I will advise for now on how to diagnose the problem without one.
Try the following and get back to me:
Pull the plastic cover off of the thermostat, if it is an OLD thermostat, with NO mercury bulbs in it... it will have a big spring looking thing inside, attached to a tiny set of electrical contacts...those will be very low voltage as a rule...if they are low voltage take a finger nail file and clean them up,,, if the gas comes on, that was the problem.
If there is no such issue with the thermostat, then follow each wire (2) from the gas valve until it runs into a control of some sort, probably a high limit controller, press the red button on that baby.
If the furnace starts it was off on the high limit... if it goes off again on that switch it willl be a defective fan, real dirty air filter, or bad high limit switch... act accordingly.
Good luck, if this set of answers and diagnostic instructions helped, please click the 'accept' button. then post back if there are any other issues, I will be glad to help further, Phil