Just the fuel gauge, or did the temp gauge also stop working?
If it is just the fuel gauge, it's probably the connection at the tank, under the seat. The connection there can get loose, and the gauge will stop registering. You'll need to remove the seat to get to it. When mine does that(and it do, every now and then), I just unplug it, and plug it back in, and it works fine. The last time I did it, I crimped the connector a wee bit, so it would hold on tighter, and so far, it's been working great....no more fuel gauge problem.
If BOTH the fuel, and temp gauges quit working, the most likely problem is with the 7v regulator. Either it has taken a doo-do(hi-tech terminology), or the gauge wiring may have come unplugged from the regulator. The regulator is mounted on the back side of the gauge cluster.