Is the security light on the dash on? If so this thread may help you fix it
http://www.automotiveforums.com/vbul...y+system+reset
Not sure even if your car has the security system, other things you could try (use spare key to see if orig is worn), does your car have that resistor in the ignition key? If so try taking everything off the key chain & see if it starts then, I believe I read somewhere on here that is caused by a hall sensor going bad or something like that, its part of the ignition, do you have a remote start on the car, those can cause problems like this.
Have you checked the engine oil? I know in some small motors (like power washers & generators) they shut off if it doesn't have enough oil, never ran mine that low in the car, so I don't know if it has one or not. Anything is worth a try if you haven't got it fixed yet.