Dead starter or dead battery,
Even if the starter works, it might not be turning fast enough to start the engine, so it could be the battery. Are all the connections clean & tight. Might be a defective starter motor/solenoid, regulator, relay, neutral safety switch, or even the ignition switch. Might be a camshaft or crankshaft sensor. If you see a security light on the dash display, the engine might be immobilized..... Beyond that, your car might have an ignition or fuel pump problem. You're either not getting proper fuel or spark. Really could be either. Without seeing the car, it's not possible to say what the exact problem is. Why don't you take it to a mechanic to diagnose. You'll then have his opinion and estimate and can have him fix it, or use his review as a guide to doing repairs yourself?