The 3 amp fuse is for the pump, not the level indicator.
The power should be coming from a permanent supply from the van.
The level is recognised by a reed switch at teh side of the cassette compartment and a float inside the cassette.
The most usual reason for failure is either the light has blown, or the float inside the cassette has become dislodged and is not doing what it should be, or the float has become stuck with paper inside the cassette.
If the float has become detached from its place it may well have been disposed of when emptying the cassette.