Possibilities:
1) the bottom screen of the gas pickup tube in the gas tank has fallen out so the float is laying in the bottom of the tank
2) the F1 fuse in the gas pickup tube in the gas tank has blown so no readings can be sent to the guage
Both problems are fixable but require removing the gas lines to pull the gas tube, etc....
Does your ski still have all the original grey tempo fuel lines? If so, those all should be replaced anyway and a fuel system cleaning would be in order including carb cleaning and/or rebuilding (with genuine MIKUNI carb kits ONLY).