wife bought me said fish finder last December I put it on pontoon in January and used it when we set some brush fast forward to August on vacation and when I turn on and it gives me my normal 3 options page

1) normal
2) demo
3)diagnose mode

when I hit the button to run normal it reboots itself and goes back through start up again and back to giving me the 3 option page again if I hit the demo mode it goes into demo mode you see running at stores and runs with no stoppage if I hit the diagnose it does go into that mode with no issues . while I was in diagnose mode I did see a voltage reading of 0.0 so I hooked up brand new battery to system with same results including the voltage reading of 0.0 . any ideas ?