Do you mean the key won't turn, or that it doesn't start when you turn the key?
My last car had the key with the chip in it and one day it wouldn't start when I turned the key. For some reason the ignition wasn't reading the little chip in the key so the car wouldn't start. My son, the car genius, taught me a trick to try if it does that - this sometimes works and sometimes doesn't. Turn the key to the "On" position, where the things on the dash (radio, clock, etc.) come on but not all the way over like you were starting the engine. Turn it to "On" and just leave it like that for 20-30 minutes, then try to start the car. Sometimes if you're lucky, doing this will cause the little computerized dooies in there to reset themselves and the car will start right up.
I only wish my son had told me this BEFORE I paid the $80 to have the car towed to the dealership and not AFTER!