After the car dies, does the theft light blink rapidly? If so, then your key is probably de-programmed or shot. You can try the other key to see if it will work.
I've had this happen to me multiple times and the last time I had to have the car towed to the dealer after trying both keys. It seems that both keys "lost" their programming and would not work. Thankfully, the diagnostic and re-programming only cost me about $100 and the tow was free (thanks AAA).
EDIT: A new key would cost about $85 each and should include the programming and cutting but each dealer has their own prices. If you have to replace both keys, then you are in a whole new ball-game.