If the theft deterrent system is preventing a start-up you will notice the theft light flashing about two times a second. The vehicle will give you three attempts to start, and what it will do is start, and then die about 1-2 seconds after it starts. After the third attempt, the vehicle will not even crank, it simply won't do anything.
If that description does not match your symptoms, then you don't have a theft problem. If it does, the first thing to try is another key, and make sure there are no other transponders on the keychain such as a Mobile Speedpass.