I had the same problem with my 6-disc head unit. I used a small plastic knife and incerted it on the right side (maybe it was the far left; it was so long ago) very quickly. It activated the motor inside and the CD ejected. Do this at your own risk. It took about 3 tries. Don't push the knife all the way in the first time. Go in a little bit at a time.
I have since replaced that head unit with a Pioneer AVH-3200. Who uses CD's anymore when you can use a 8GB or 16gb USB or SD card.