Glad that yours fixed itself. When this happened to me, I did a lot of searching for solutions. Several people did have luck with doing a "reset" and having the system come back to life - just removing the battery cables and then putting them back on.
This didn't work for me, and so I had to remove mine and pull the CD's out by hand. I bought some of those U-shaped pullers and could not get them to work, but removing the plastic that surrounds the stereo and heater controls revealed that you can pull it out that way too.
I followed this video on how to remove the CDs. . Towards the end, he shows where a gear falls out of the unit and you have to manually put it back into place. This was what happened to mine, but even with it back in place, the Error message was displayed. It may have been something that I did while inside the unit - this thing is so fragile and complicated that I'm surprised they last even one year. So, having one that survived 18 years wasn't a bad thing.
It's easy enough to hook up a $20 Bluetooth unit and connect your phone. Or spend $100 and buy a new stereo.