Have you tried switching slots with your map card? I had a similar situation last week and think either the slot went bad or the blank card went bad. Showed the right lake, correct outline, my position on the water but no contour lines. Even locked up my unit.
My daughter tells me that storage cards can go bad from numerous on/offs or too many write/erases. She is more savvy on this tech stuff than me so I'll mostly believe that. My unit is a G4 Helix 9, about 3-4 years old. Leave my blank card out and unit works correctly, even with my map card in the other slot.
You might also try putting the cards in and out a few times to sort of clean the contact areas if they don't get moved around much.
Yes, I was talking to myself; sometimes even I have to ask for expert advice.