First start with the easy things. Make sure that your key batteries are ok. If you look at the red plastic piece on the key in the dark, if you push either the lock or unlock button you should see a brief faint flash of light. That means the battery is ok.

In my case, the problem was intermittent - if I waited 5 minutes sometimes it would start. Now I also noticed something else (you're going to call me crazy) - trying starting it with the door open and with the door closed. I swear that it may make a difference, at least I thought it did with mine - it would start if the driver door was open.

I hope you get it started.