I discovered last night that my bike (1988 Yamaha SRX250) has developed a strange problem: The headlight doesn't work when switched On, and in low beam. The dash lights up and the tail light goes on, but nothing shows up front. When I switch to high-beam the light comes on and everything is normal. The way the switch blocks are set up on this bike is with a light switch on the right switch block with "ON, PO, OFF" settings, and a "HIGH, LOW" setting on the left block. I've had a fiddle with all the controls and I'm sure I'm not just pushing the wrong button. Any suggestions on what might be wrong and how to fix it?