My guess would be one of the two following situations:

1.The transponder in your key is acting up, try your other key like bwheitman suggested.

2. Check your battery. If the voltage is too low it might not activate the starting circuit.

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Ray
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles