If the central air conditioning unit freezes up, locate the high-pressure lines that run from the condenser to the evaporator coil. If the lines are covered in ice, disconnect the system from power completely by switching off the breaker in the panel. You can also find the disconnect switch located on the outside of the unit next to the condenser, and pull it. If that didn't help, contact a professional to add more Freon to the unit.
If the air conditioning unit doesn't work, make sure that the unit has power by checking the circuit breaker and the fuse box. In addition, locate the disconnect box on the outside of the unit next to the condenser and make sure that the fuse in it isn't blown. If that didn't help, thoroughly clean the furnace filter or even replace it if the old one has significant signs of wear. Check if the condenser coils have dust or debris on them, and wash them using a garden hose. If the unit still doesn't work, call a professional.
If the air conditioning unit doesn't turn off, set the thermostat to a higher temperature, make sure that there is nothing stuck in the condenser, and clean it. If you have an older conditioner, oil its motor. Clean the furnace filter and blower to get rid of any dust or dirt.