1999 audi a4 2.8

hey guys,
i tried to change the battery in my car.. didnt worry about having a consistent trickle of electricity because i have the radio key on hand. I replaced the battery and the car starts up fine..... but cuts off within 2 seconds of being on! i read that my model has no immobilizer but it seems like thats the problem... any help, ideas, would be greatly appreciated. i lostone of my jobs recently so im hoping i can get it on the road on a budget.. thanks again guys