If any breakers are tripped, reset them once. If they trip again do not reset them. Deadly high voltage conditions exist inside the air handler cabinet and inside the access panel of the condenser. Let a qualified serviceman open them. If the air handler runs constantly but cannot satisfy the thermostat setting, it is possible the backup heat is running but the condenser is not.
Some condensers have the high pressure cut-out switch externally accessible.
Look for a button sticking out of the cabinet in the vicinity of the refrigerant pipes. Press it in. If the machine starts up, the head pressure got too high, possibly from turning on and off too quickly, too much or too little refrigerant, or an electrical interruption. As the unit ages, the switch can weaken and pop easily. If the condition repeats itself often, have a serviceman check it.