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The fuel sender's float filled with fuel or perished, so it no longer floats. A sunken float reports empty regardless of the actual level, which is the safe direction for the driver and a nuisance for everybody else.

The reading is constant rather than erratic, which distinguishes it from a worn resistive track — that one is wrong over part of its range and correct elsewhere. Watch the reported level across a full refill: a value that never moves is the float, and one that jumps or sticks over a band is the track. Ethanol-blended fuel is hard on older float materials, which is why this appears on vehicles that were fine for years.