1991 Nissan Sentra automatic transmission

Check all fuses to make sure they are ok. If they are ok disconnect the starter motor solenoid wire and probe with a test light. Have someone try to start vehicle. If test light lights up turn engine over by hand. If ok then replace starter motor. If test light does not light up then further fault tracing is required to check for a bad relay or faulty ignition switch. Carry out these basic steps first and if you need to go further I will post some wiring diagrams or have vehicle towed to a shop for proper diagnostic.