Had this problem and was going to try to pry the CD out with tweezers, but turns out I didn't have to. I had tried turning the car off hoping it would reset, and even removing then reconnecting a battery terminal as suggested on another thread, but neither worked. So I used a credit card to pry and hold the door open, but when I looked in I couldn't see anything to grab onto with tweezers. So I thought hitting eject at that point might at least move the CD into sight, but when I did that with the door held open, the display went a little nuts, ERROR 4 changed to EJECT ALL (which I've never seen before) and it spit my CD out!
Don't know if this will work for anyone else, but it couldn't hurt to try.