Check your home's circuit breaker box for a tripped circuit if the air conditioner won't come on. If a circuit breaker has tripped, simply reset it.
Make sure the unit's thermostat is set to cool and is set at the required temperature. There can be dead zone in the thermostat where the air conditioning won't come on if the ambient temperature and the set temperature are too close. Set the thermostat to a different temperature until the air conditioner kicks in. Try turning the thermostat off for a few hours to see if the system resets.
Clean any filters or replace them (if your owner documentation advises replacement) if there's no air flow. Turn the thermostat off for a few hours--the coil may have frozen up. Water under the unit with a cold exterior surface on the indoor coil enclosure is a dead giveaway that the coil may have frozen: the feezing is caused by a dirty coil.
Check for constant cycling if your utility bills are unusually high. Check that the thermostat reaches its set point---if it doesn't, call for service. If the equipment runs all the time, try changing the set point---you may be asking too much of the air conditioner and the set point may be too low.
Turn off the equipment and contact your dealer if the drain pan overflows; it's likely an overflow switch problem.