All of a sudden, my central a/c quit working. When I turn it on, neither the inside nor outside unit will come on. If I set the fan switch to on, that doesn't work either. Any ideas?

Check the breakers going to the inside unit. If it is marked heat or furnace at breaker box turn it off and then back on even if it looks like its fine. A lot of service calls are as simple as resetting the breaker. If resetting the breaker doesn't work you may have a blown low voltage fuse or transformer inside the furnace. The newer systems will give you a flashing light at the furnace to let you know what's wrong.