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First disconnect the battery (or pull radio fuse) and hope the changer resets and ejects.
Second try gentle rubber hammer vibrations while trying the eject button to shock the changer into ejecting the CD.
Last hate to say...remove the unit and start disassembly to find the release switch mechanism.
Good luck to you.
I don't what CD player you have. I have the NAV with six disk in the trunk. Several times in the last few years I've had a similar problem and each time disconnecting the battery worked. Good luck.