I have about a 5 year old heater in our home. The thermostat clicks like it working (digital display) shows the flame like the heat is on.. but the unit in the attic doesn't come on. No fan or anything.

I went in the attic and there is a light switched hooked up to the unit. I turned it off and back on.. and the unit fired right up.

But when it turns itself off.. it will not jump back on when the thermostat clicks and wants to go back on...

Any ideas?

The switch the you clicked off and then back on reset the PC board in the unit and it worked because the thermostat was calling for heat. But the next time it called for heat something malfunctioned in the heater that made the board lock out again.
There are several things that can cause this problem, the first I would check is for a dirty filter, next I would clean the flame sensor, I would have someone turn on the thermostat while you are in the attic to see which one of the phases the heater might not go through.

Thanks letmetellu..

I will see about checking this stuff today. As for filter, you mean the one inside the house? I change out both of them every month. I just did it on the 1st..