Set the thermostat of "off" and replace batteries. This establishes communication between circuit board in your furnace and thermostat
Replace filter(s) at return. An air conditioning systems needs to have correct air flow passing through system.
If you see water on the ceiling first thing to do is turn "off" AC. AC system produces many gallons of water a day especially in the summer time. Possible clog drain lines.
If you see ice on the copper line outside at the condenser, turn "off" system. Possible refrigerant leak, dirty filters or fan motor.
"what went wrong"
Well your air conditioning is not working. What to do now? There are some simple things that you can do that can possibly fix your air conditioner. Of course, this can be done at your own risk, but can save you time and money. Here are a couple of simple things that could be wrong with your air conditioner or heater. Sometime following these steps can get your home comfortable again.