You might want to take a few minutes to make sure you don't have a pinched feed wire anywhere -- any short in the wiring will pretty much cause everything to crash.
Had the display worked with the GPS' sender and the depth sounder's sender worked with the GPS, that would've been my next check. Turned out to be unnecessary.
Don't ask me why I know this...
That is a concern on Abracadabra, though. The past owner(s) have not been good about routing wires and stuff so as to avoid them becoming victims of stuff "wandering" about in the storage spaces. I've known this was something I needed to address, but there was always Something Else. Enough!
So, here's what it was: Whomever installed the transducer left the O-ring off. Result: Over the years, as the boat heeled, a little bit of the mineral spirits bath in which "puck" sits kept leaking out, until, eventually, enough leaked out that the transducer stopped working. Mopped-out the remaining mineral spirits, re-filled with the prescribed amount, installed the O-ring from the unused sender. Fixed!
While I was at it: I checked and found the sender hadn't been installed properly. It was shooting off at an angle to starboard. Fixed that, too.