I'll bet 9 out of 10 people whose fuel gauge is out have a clutch switch that is stuck, causing a constant connection. It probably starts with a bike that won't start because the clutch switch is corroded and unable to make that connection. In the process of cleaning it and putting it back together you may end up with some kind of bind that prevents the switch from releasing. Your bike now starts, but your fuel gauge is kaput.
Moral of the story- If your fuel gauge no longer works, and you can start the bike in neutral without pulling in the clutch lever, you need a new switch (or you can try repairing the old one).
I have a new/used switch coming from Pinwall. I predict that if the bike starts, but only while pulling in the clutch lever that the fuel gauge will magically come back to life- we'll see. I also expect for $50 to gain at least 20 more horsepower 
Seriously, from what all I'm reading it just may actually run better as a result.
