I bought one of these at a yardsale and when I got it home, it had this same problem -- just the Tone A light and non responsive. Unrecognized by my Mac and iPhone app too. I took it apart (just 4 hex screws on top and 4 on the bottom - will void the warranty). Make sure power is off. There's a micro flash card on the main board. I just had to pop it out and put it back in. I then powered it up and it works fine now. The card was a bit hard to get out, probably a dot of glue on it.
Hope this helps
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