We have Rock Band 3 for Wii, with the Mad Catz Rock Band Midi-Pro Adapter, and a Yamaha P-95 keyboard. This was working just fine. We hadn't used the keyboard with RB3 for a while, and when we plugged it back in, no go.
The buttons on the controller itself work fine to talk to the Wii and control the game. When we switch the D/K/G switch to "G" (just trying things), it blinks the S. O. S. pattern. But when it's on K, the MIDI signal light doesn't blink. (And, of course, no actual keys received in the game.)
Crucially, this was working. It's not like the thing has batteries. What could have gone wrong, and what might we do about it? The manual is no help. I also looked in the Yamaha P-95 manual, and although I didn't need to do it to get the adapter working initially, I reset the keyboard to factory defaults and tried variously setting the MIDI out channel to 1 and 10.
What else can/should I do? New ones are $35, so I could just suck it up and get a new one, but I don't want to do that if it's not likely to even solve the problem (or if it could be fixed in some other way).