My original Onkyo six-disc changer displayed this error message after about 20 years of solid service. There was also a grinding noise when I hit the open button. I had to unscrew the back and sides to get my CDs out. The rotating tray that holds the CDs appeared to be jammed halfway out of cycle, which was preventing the shelf from opening. Current prices for the same machine are about $200 less than what I payed for one in the late 90s, so I bought a new one.
If your's is still in warranty, bring it in for service.