The most likely scenario is that the stepper motors that move the needles are dead. Happens often on these gauges. The Malibucrew talks about a DIY option to replace these. Definitely try plugging in the gauge to another plug first before opening up the gauge to replace these small stepper motors. You can find the stepper motors online for cheap, they are used in cars as well. I would recommend replacing them all if you go the DIY route.
Also, watch when you turn the key one click to the right, these needles should vibrate a little. There is also a way to test the gauge functionality by pressing the speed adjust button while the screen displays lake temp, or some combination. Someone here might have the exact process. I'm not near my boat now to verify.