My heat or air is not coming on. It's says there is no ac power on thermstat. I have looked for fuses on the upstairs unit but could not find anything, need to know if this is the problem and what could it be and where the fuse is. Thanks for your help

What brand and model of furnace. Many are diffferent so I would have to guess at 50 models per year x 20 years. That is a 1000 different units. Lets see if we can narrow that down a bit. Model and Brand of furnace please.
Model and brand of thermostat please.