Air/Heating System Stops Working Intermittently

For months, my air conditioning would just not work some days. When I say not work, I mean absolutely NOTHING happened. No air, hot or cold, at any speed low-high. Coming into Winter, it continues to do this, which is not handy on cold or rainy days and no defrost on my windshield. I had it to a mechanic, for a blower motor, assuming that is what it needed. The mechanic happened to get it on a day it was working, so he told me to bring it back when it does it again since it cannot be the blower motor if it works here & there. I do not want to take it back to the garage again for a guessing game, and lose more money. However, If I need to bite the huge monetary bullet of taking it to the dealer for repair, I will. Can anyone help point me in the right direction of what this is????