Look for repairs
on the internet
. I had a friends Raymarine chartplotter
go black somewhere in Italy
. We followed the instructions found on the internet
for taking it apart, cleaned all the contact areas and put it back together. Everthing works like a charm since then, 3 years ago. Contact oxidation seems to be a known problem, just try it, nothing to loose it is winter you have all the time. We were moored when we did it in one afternoon and with some rosť wine.