Before calling for service, check these simple fixes:
Step 1 - Thermostat:
Is it set to "cool" not "heat" or "off"?
Is the temperature set below current room temp?
Are batteries dead (if applicable)?
Step 2 - Power:
Check circuit breakers (indoor and outdoor units)
Look for external disconnect switch turned off
Step 3 - Air filter:
Is filter completely clogged?
When was it last changed?
Step 4 - Outdoor unit:
Is it running or completely silent?
Is area around unit clear of debris?
If these don't resolve the issue, schedule service. Avoid repeatedly turning the system on and off, which can cause compressor damage.