Hi, Does the gear engage and mesh? All you might have to do is rotate the spindle slightly by hand to get it to engage. If you tried that already and it doesn't help tilt you head and take the back cover off.
You will be able to see the 'gear train'. Your problem sounds like a 'finger' to 'gear' mis-alignment. With the head tilted, power off, and cover off, move the high low gear 'knob'. If the finger is not moving it, a few different things may be wrong and you'll have to work through it step by step, could be with the 'knob' itself, the 'sliding gear', 'key', 'keyway', etc.
I'm pretty sure once you tilt the head and look you see right off where the problem is, it'd also be a good time to grease the gears too, the factory is negilgent sometimes in this respect (; Don't go overboard with the grease. You'll want to use grease with some 'cling', and lithium based should be fine.
Hope, this helps, if not and you take some pics, the problem may be easier to isolate.