The anti-theft kicked in. Mine did it a few months ago and It had something to do with my Superchip. I had to return mine to stock and then re-install the program. Do you have any chips/programmers installed?
It has something to do with the PATS system installed which reads a code from the key. Do you have a second key you can try?
If not, Try this. It works for me:
THEFT LIGHT (on Excursions) - If the theft light flashes rapidly after programming and the vehicle won't start, the Passive Anti-theft system (PATS) will need to be reset. To reset the PATS, perform the following steps:
1. Turn the key on, but do not attempt to start the vehicle.
2. Wait for the theft light to quit flashing rapidly. This should take about 45 seconds, after which the THEFT light will flash multiple times about every 3 seconds.
3. Turn the key off and remove the key from the ignition for 30 seconds.
4. Insert the key and turn the ignition to the ON position.
5. The THEFT light should illuminate for about 5 seconds and then go out.
6. Continue to start the vehicle normally.
Hope this helps....works for my 03 X.