I have a problem on my furnace. My first level won't heat. Was told it was the power head change power head and still have the problem. Change thermstat and still have the same problem. My second level I have no problem when I ask for heat.What could it be?

When you say furnace you imply hot air. Power head tells me you're zoning the system which is not really typical of hot air.

Is this a boiler? If it is, make sure that there is no air in the system by opening the bleed valves at each radiator until you get a solid stream.

If it is hot air, is the burner coming on and the blower running?