As others said - first try the simple fix of removing each wire on top of the tank and cleaning them. Be sure to clean the stud on top the tank as well. Double check the ground wire - IMHO , 90% of 12 volt problems are related to bad ground.
If that does not fix it - try gently flexing the wires , one at a time - while a helper watches the gauge.
Some times - corrosion creeps up the wiring. If flexing causes gauge to work - then you'll need to pull a new wire.
Most likely though - the float has failed. The new E -10 fuel can degrade the adhesive on the float and it fills with gas and sinks.
You can check this by removing the sending unit. Some are like a tube - leave it all connected but tilt it so float moves up. Watch the gauge. If it works - bad float.
Others use an arm with a float - just gently move arm upwards - see if gauge works.
Tip: Be sure to really clean top of tank before working on it so no loose crud / dirt gets into tank.
Watch the screws and tools - amazing how they can fall into the hole.
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