The air conditioner thermostat

Above are several wall thermostats

The temperature should be set well below the temperature of the structure.  
There should be a system switch on the thermostat.  It should be set to "cool".  If you are unsure of how to set the thermostat, click here for instruction sheets for different thermostats.
If your thermostat has a digital read out and it is blank, remove the thermostat from the wall and replace batteries.  If you still have nothing, there is probably no power to the system.  If so, go back