Please check the HVAC circuit breaker.  If the breaker is tripped to the OFF position, please flip the breaker to the ON position.  If the breaker is still in the ON position, please turn the breaker to the OFF position and then back to the ON position.  Please give the unit several minutes to determine if the Heat or A/C will become operational.
Please check your HVAC filter to determine if it is dirty.  If the filter is dirty, please replace with a new & clean filter.  If the unit has ice on the outside unit or on the heat exchanger / furnace (located in the attic, basement, crawl space or furnace closet), please turn the thermostat to the OFF position and turn the fan to the ON position as this will assist in defrosting the unit.
Please check to make sure the heat exchanger / furnace switch is in the ON position.  Most heat exchanger / furnace will have a switch near the unit that usually mimics a light switch.  Occasionally this will be turned of inadvertently by a tenant who mistakes it for a light.  Turning this switch off, eliminates power from going to the unit and therefore, renders the unit powerless and inoperable.
Please check the batteries in the thermostat and change if needed.