CD stuck in CD player

You can try to reset the unit by disconnecting the battery negative terminal for a minute and see if it will reset long enough to get the CD out after you reconnect. Do not turn the radio on, just reconnect the battery and press eject to see if it will spit it out.
The only other way to retrieve them without damage to the CD itself, would be to remove the CD unit and dismantle it enough to get the stuck one out. Normally this is just the start of problems with those units. The common issue is that the plastic guides break inside so the discs do not set flat and they will not play or eject.
There are places that repair them like United Radio and others around the country, or you can get a used unit from a salvage yard or just replace it with something better.