Is it the bulb, or the switch?

 

Have you done any other work lately (wiring or otherwise) that coincides with when this problem started?

 

 

You will need a wiring diagram from the back of the service manual to diagnose the problem, it should be very simple as there is a neutral switch on the transmission, the wiring to it and then up to the dash, and the bulb itself.
Depending on the year the power may go first to the switch then up to the light and to ground, or sometimes the power may go to the light first then travel down to the switch and go to ground from there.
Either way once you consult the proper wire diagram for your bike you can use a multimeter to trace power and determine if it is a wiring issue, switch issue or bulb issue.