Car would crank but not start what can be the problem?

I have a 1996 Pontiac Grand Prix that's been sitting in my garage for 7 years last time I drove the car it was over heating alot so Its just been sitting there. I've decided to fix it but a mechanic came and messed up the ignition switch so I had to buy a new one, I also put a new battery in and also put gas the car would crank but not start also the security light on the gauge cluster is on, what could be the problem?