99 Cavalier won't start

My 99 2.4 Convertible Cavalier won't start. It turns over, but won't start. Sometimes, but not always it makes a weird buzzing sound when trying to start it. It had been giving trouble starting for about the last month, but it always eventually started. Sometimes it won't start after sitting all night, sometimes after driving it, running into the store, and then won't start when you come back out. It did this about three months ago also after running out of gas and we put a used fuel pump on it and everything worked fine. Now it just won't start. Does anyone have any suggestions? How much does a diagnostic test cost? I am limited on funds and will have to have car towed. It has new spark plugs.

have the ignition moduel tested this is located under the coil packs