how can i fix my in car cd multi player/cartrage stuck?

I have a P38 Range Rover with a Harman Cardman
x6 multi play CD changer.
when I load it up and insert the cartridge into the changer it goes through the motions of loading up all six cds, but does not play them it!!
when I press CD button on radio it always says NO DISC, what could be the problem and can I fix it.

Sheila
March 2014