If your home air conditioning won't switch on, your first thought may be to call the repair man. However, you can identify and fix a common problem yourself. Try these steps before you spend a fortune on a professional job.
Check that the thermostat is set to "cool."
Check if you have a blown fuse. The air conditioner should be on a dedicated 240-volt circuit. If the problem is here, reset the fuse. If it still doesn't work, go to Step 3.
Check that the furnace power switch is turned on, then check the switch on the outdoor condenser. If they're both on, go to Step 4.
Switch off the power to the air conditioner.
Remove the cover from the thermostat.
Unscrew the wire from the Y terminal using a compatible screwdriver. The terminal should be labeled.
Hold the wire by the insulation and touch the bare end to the R terminal (which should also be labeled) for two minutes. If the air conditioning starts working, it means that the thermostat is broken. If it doesn't, the problem is with the air conditioner itself. Contact a repair man.