Check your breakers to make sure they are ON. If OFF, flip back ON, and then check if your system will run within 5 minutes.
For gas furnaces, turn the breaker OFF and then back ON. Wait 5 minutes, and then see if furnace will start.
Below are additional steps you can perform so that when you call I can have a better idea of what the problem could be, and thus, give you better service.
If the air conditioning or furnace is in your attic, check to see if the emergency drain pan is filled with water. If water is present, then the drain line is clogged and we will need to unclog it, but the system can work temporarily if you remove the water from the drain pan.
At the thermostat, switch the fan to the ON position and see if the indoor fan runs (Can you feel air moving through the vents?).