When you call an hvac company you are almost always asked If you know the nature of the problem with your system. The reason you were asked this is is to help the technician know what parts to bring with him to fix your furnace, air conditioner or boiler The following is a shortlist of ways to diagnose what the problem is with your furnace or AC so you can help your technician know what to bring with him or her.
1st. Try to know the make model and serial number of your unit
2nd. Check for power. Be sure you have a live circuit and that the breaker is powered on.
3rd for ac check for air flow and a clean coil.
4th for ac verify that the outdoor unit fan is functional.