Here are some simple procedures you can perform before opting for the expense of a service call:
Check disconnect switches (indoor and outdoor if you have a split system).
Make sure that circuit breakers are ON and that fuses have not blown.
Check for sufficient airflow.
Make sure air filters are clean and that supply-air and return-air grilles are open and unobstructed.
Check the settings on your thermostat. If you want cooling, make sure the temperature control selector is set below room temperature and the SYSTEM switch is on the COOL or AUTO position. If you want heat, make sure the temperature control selector is set above room temperature and the SYSTEM switch is at HEAT or AUTO.
Make sure the FAN switch should be set at AUTO.
Click here for a detailed guide on troubleshooting your AC unit.