Car will turn over but it will not start

I bought this car (turbo) from a guy. He said the car ran fine up until he went to change out the stereo and put a after market one in its place. He said when he done that it wouldn't start anymore. I put the stereo back in it that was originally in it and still the same thing. It will turn over over but it still will not start. I have also noticed that the full pump isn't coming on. I am guessing it could be something to do with the security anti-theft since he tried changing stereos but not for sure. I could really use some help and advice about the problem I am having. It is my only car and I really need it up and going. Please help me.