When you push the handlebar switch, does the starter solenoid click?
If yes, the handlebar switch is fine. If no, the switch on the handlebar is either disconnected, 12V not getting to it, ground wire from it not grounded, or switch is broken, or the coil in the starter solenoid is disconnected or bad.
When the solenoid clicks, is there 12V on the Big Lugs, both sides? one side is Batt voltage, the other is switched.
If you don't have 12V on both sides, the solenoid is bad.
If you do, then check for 12V at the starter lug when the solenoid is engaged.
If you have 12V at the starter, and it isn't turning, then the starter needs some love. Could be broken wire off 12V lug, which is common if you don't use 2 wrenches to keep the stud from turning.
Could be bad brushes that finally went.
Follow the power. 12v to solenoid coil, then solenoid coil to handlebar switch. Handlebar switch grounds wire, solenoid closes.
12V from batt to large lug on solenoid. Other lug on solenoid to heavy wire to starter...
If you follow the power, the problem is where it either doesn't have power, or has power and nothing is happening.