If the oil tank gauge has failed, it should be replaced by a certified technician. However, before replacing the unit, test it and ensure the component is broken.
An oil tank gauge works using a float assembly the read the level of the oil as it is being used. In order to test the gauge, unscrew the cover from the assembly.
Use one finger to push down from the top of the gauge rod indicator. The healthy unit will have some resistance, and will slowing rise up afterward. If the meter does not rise back up, then it is damaged and will require replacement by a heating technician.