Do you have a digital gauge cluster or are they analog gauges?
If it recently stopped working I would first check to see that they are getting power by checking fuses and then probe the leads right at the back of the gauge itself.
Next make sure that the little spedometer wheel at the back of the ski spins freely and is putting out a pulse while rotating as measured at the gauge.
If everything checks out and still no reading on the gauge then the gauge probably has corrosion issues and will need to be taken apart and fixed or replaced. Faria is probably the manufacturer of the gauge and they offer a service to repair broken gauges.
Check the speed sensor on the bottom of the stern...I bet the wheel broke off....it's an easy replacement...search on ebay for a replacement wheel, otherwise the whole sensor will set you back $90-$100