If it was an aftermarket alarm, or even a factory alarm.
Open up car, quickly look for a small momentary switch button somewheres around the driver's kick panel or general driver side area. Depress it while having the key to on (keep your finger on for more then 5 seconds).
If this works, the Alarm will disarm and you should be able to start the car.
This is only one way of dealing with an alarm. I admit I'm a bit sketchy on factory alarms, not sure if they have the momentary switches.
By the way, the switch should look like this:
http://www.radioshack.com/product/in...ductId=2062496