Open it up
Assuming your unit is off warantee.
Although the screws (Torx key) are hard to find, you can use a metric hex key to open it up. There is a very small ribbon cable inside that can oxidize and loose connectivity. You can disconnect it very carefully by prying on the white security tabs on both sides. Use contact cleaners and a pencil eraser will bring it back to life.
Make sure you get some di-electric compound and smear on the rubber seal when reassemble. That will keep the mositure away.
Don't ask me how I know. If you are interested, I have a pair of spares (hardly used) Depth 3100 and Speed 3100 in perfect working order for $100 a piece if you are interested.
my e-mail is "alexco38 @ gmail.com" just remove the spaces.