Mine did this periodically. Hold eject and wriggle the magazine. Eventually it will spit it out. You might need to try this with the ignition off, that's what worked with mine.
Once it's out, dismantle the magazine and Mr Sheen the components. Mine then worked fine for ages after doing that (?>1 yr IIRC)...although it subsequently died completely :headbang:
I suspect the CD changer cannot tolerate the harsh ride of the ///M supspension on crappy uk roads. As srhutch has said, an Ipod connection is the way forward