The computer in my car thinks someone is trying to break into the car so it wont start!?

I have a '91 New Yorker. It has a computer, but does not have a security built into it HOWEVER all of a sudden it thinks that it does. It believes someone is breaking into the car so when I start it the engine cuts off. The mechanic says it'll cost $2,000 to fix. Anybody know a way that I could fix the problem myself? Thanks!