The first thing I would try is to see whether rolling the bike while in gear and the clutch fully engaged (i.e. let out) turns the engine or not:
- If you put it in first and roll it but the wheel drags or turns the engine then you know that the mechanism that transfers power from the engine to the back wheel is at least partially in place, and that the clutch is at least partially engaged.
- If the wheel spins without resistance then you know have a problem somewhere between the engine and the wheel: the clutch is not engaged, the transmission is busted or you have a problem with the drive belt
From your description it sounds likely that the wheel will just spin while in gear, if that's the case I'd trace the clutch cable to see if there's any obvious defects like breaks or kinks, then I'd have a look at the drive belt. Anything more than that's going to be a bigger job and you'd want to think seriously before tackling it yourself.
EDIT: apparently the wheel doesn't spin when in gear but does spin when out of gear or the clutch is disengaged. The drive belt is fine, and it revs when out of gear.
According to this forum post about losing the gear indicator there may be no gear position sensor, instead it could be a problem with a switch or sensor, and it could be tied to the loss of power - the ECU may not be supplying power because it isn't getting information. I would suggest you read the diagnostic codes from the OBD port to see if there's a clue there.