Check your circuit breakers and if the switch on the side of the furnace is 'on'.
Check your furnace compartment door to insure it is closed. The unit will not come on if the door is not completely closed.
Is your furnace filter clean? You must maintain a clean filter even during the summer season.
Changing your thermostat temp rapidly may cause the compressor motor protector to open. When this happens the compressor will turn off for a short period of time. Allow 15 minutes for compressor to automatically reset itself.
Does your thermostat need new batteries?
Check outdoor condenser for any blockages. Enclosures around your outside unit can interfere with proper operation. Rinse the outdoor condenser off with a garden hose. A light rinsing (careful not to damage fins) can greatly increase airflow to the condenser allowing it to breathe.
Sometimes you may experience rooms cooler or warmer than others. You can balance airflow to your satisfaction. Simply pull the lever on the registers downward to decrease and upward to increase airflow. Remember, when increasing airflow for one area you decrease in another. It is not uncommon to find the need to change the airflow for each season.