The mechanism may be jammed or some gears stripped out... this is common with the in dash changers, particularly the Mitsubishi built ones. Often, the only thing you can do is take the whole thing apart to get the discs back and then write-off the changer.
In the lower corner of the faceplate should be the radio sales code. It will be 3 white letters beginning with 'R'. I suspect it is a REU if it has NAV and 6-disc? Was the battery disconnected or go dead and/or was jump started?
A battery disconnect,pause and reconnect may reset the disc loading/unloading logic.
Press and hold the EJECT button for five seconds and all CDs should be ejected from the radio.
RHR: forced eject, open radio display, hold the nav button, press the seek up and seek down at the
RER : forced eject, open radio display, hold the nav button, press the seek up and seek down at the
REN: forced eject, open radio display, depress the audio button, press the seek up and seek down at
the same time.
Otherwise if you don't feel confident about disassembly and repair, see your dealer. They may send the unit out to the repair depot to extract the discs and you will get your unit and discs back.
I have gotten the disc magazine to unload the discs by manually turning the loading/unloading motor gear.