How can I fix a Sony CDX-C5000R? Buttons don't respond?

The buttons and dial on the facia panel of my Sony CDX-C5000R are not responding, except for the power on/off button. A CD will play, but I can't alter the volume or anything else. I have tried the reset button and I have tried cleaning the contacts - no luck.
Can anyone help or suggest next step to find/fix the problem?