Trace YCC won't blow hot air

[firemancyr]

I've got a Trane YCC42F Combination Cooling/Gas Heat HVAC unit. Worked fine all last winter (my first winter in this home) and this winter up until last night. We woke this morning to a heater blowing cold air and a house in the mid-50's degrees.

The unit is located on my roof. I turned off the thermostat in the house, went up, opened the unit, and had my wife turn the heat on while I was at the unit.

The fan comes on, but I get no other action. No gas flow, no electric ignitor clicking, no flames.

Is there a simple solution to this or do I need to call a tech?

[HayZee518]

is this a heat pump? if it is a natural gas burner, check the flame detector, the air/gas mix orifice for bugs or spiders, the lockout relay, purge fan if there is one, pilot orifice, thermopile generator (in place of the flame detector rod) no pilot light - main valve won't open.