Try tapping on the starter with a hammer and see if it starts. If it does, the starter solenoid is faulty and it's time to replace the starter motor assembly. Another possibility is the motor is locked-up, or there is no power getting to the starter solenoid (possible relay, neutral safety switch or circuit issue...or blown fuse or fusible link). NICO Club's website has Nissan factory service manuals in their database that you can access for free. Step-by-step diagnostics and wiring diagrams are located in the Starting and Charging System section.