Since this problem started, I have to turn the switch of and on if the heater wasn't working. Sometimes it works fine for a day or two and then I have to switch of and on when the heat won't turn on. I watch the furnace one time and found out that pilot won't kick in, no fire and after few more seconds the blower turns on without the heat. My unit is Lenox. Can someone please tell me what's wrong with my unit?

Take the cover off the burner compartment of the furnace. Obtain the make and model of the furnace off the rating plate in the burner compartment and post it here.

Observe the sequence of events that occurs at the furnace when you turn up the thermostat and post that sequence of events in order and in detail.

Thanks for responding SeattlePioneer

Maybe this will help.
M/N G20Q3E-75-7
Lennox: WhisperHEAT

If it works properly:
Green light goes on, after few seconds pilot strikes few times, after few strikes flames goes on, pilot stops, air blows after few seconds. Cycle completed.

If it fails to work properly (won't generate heat):
Green light goes on, air blows after few seconds. After a minute or two air stops. Cycle completed.