Why is my mercedes c240 fuel gauge not working?

If the fuel gauge on Your Mercedes-Benz C240 isn't working, the first thing I'd do is check that the fuse isn't blown. That's going to be in the fuse box inside the cabin, located below the passenger side door. You'll want to check fuse number F5, which is a blue 15 amp fuse. Even if this isn't blown, go ahead and pull it out, wait a few minutes, and put it back in; this should reset the fuel gauge. The next thing that could be wrong is a dead sender (sending unit failure). This means that the gauge isn't getting any indication of what's actually in the fuel tank. This is going to be a bit more difficult to debug; beyond that, too, the actual gauge on the instrument panel can fail, though it's pretty rare, and You'd likely see Your other gauges acting erratically too.