We have a Trane XL20i Air Handling System. Her name is Christine. We just learned a 115-dollar life lesson and while I am ok with that, I thought I would share. When you run the Ac then the heat soon thereafter, at this time of the year, it can cause a low-pressure issue with the Heat Pump and A/C operation. The valve will sometime "quit" and the outside unit will simply stop working. The inside unit runs fine, but you aren't making any cool or warm air.

The easy fix:
In your breaker panel, you should have 2 each heater circuit breakers one each AC braker. Shut off both theater circuit brakers, count to "Thousand 60" and close the breakers. If your system starts running, then good. Most of the time this will work. You are good to go.

if it doesnt then call for repair. Ousr shut off Friday night and we left it off all weekend. Kind of pissed I didn't know this, but I am also willing to pay for a life lesson. Anyway, hope this helps.