When you apply power to it with the disc carousel door completely closed, does it go through a startup routine where you hear what sounds like its loading each disc tray? The process takes about a minute to complete. The door has to be completely closed. If you don't hear that happening then I think you have a faulty player right off the bat. After the cycle completes then you should be able to open the door, wait a few seconds and then carousel should eject automatically.
Try turning it on its left end when looking at the front/door side. Then with power applied open the door. If there's a magazine tray stuck in there (happened to me twice), gravity will help it reload into the magazine, which should then pop out normally.
This isn't my idea, but I can't remember who posted it so I can give him/her props for it!
I ran into this problem with a couple of units that I bought off of Ebay. Yes, there is a manual eject button. I can't remember the details at the moment. I'll have to take a look when I get home. If I remember correctly, there is a small hole in the upper left corner that you push a paper clip into. This presses on a switch that will eject the cartridge. The door needs to be open and the unit needs to be powered. It doesn't matter what orientation the unit is in. This unit will work sitting flat or on it's side.
If that doesn't work, pop the covers off. There is a lever inside that is impossible to describe here that you push on to release the cartridge. I had a problem with one of them where a disc got stuck inside the unit but I can't remember how I got it out. Had to pop the covers.
One of the units that I bought had a small piece of plastic floating around inside that broke off during shipping because the cartridge was not pushed all the way in. This piece came off of the door and was needed so the unit could detect the door being open. Superglue and JB Weld for support and all is good.
I'll recheck when I get home but I think this is correct.
My Pioneer 12 CD changer is the original in my 2002 vet, never had a problem until now. I could see slot 7 tray would not retrack back into the tray, so the tray would not pop out of the changer. The little button in the upper left corner does cycle the unit to try and reload the CD's, but with one stuck, it would not work. I had to remove the unit from the car (6 screws - 3 on each side). I removed the top and bottom case of the CD player (6 more screws). This will not give you access to the CD tray holder, but I could clearly see a CD was stuck on the platter. I connected the unit up to the vet again and hit the reset button with a paper clip. This time I could clearly see there were two disks on the platter. I kept hitting the reset button while turning the unit. Turning the unit upside down made one of the disks fall back into the holder. With only one disk on the platter I could now see the unit move up and down against the tray. I kept hitting the reset button until the unit moved up to the top slot - tray 12. Once there I reset it again and the tray popped out. I unplugged the unit and was able to remove the stuck CD from the platter through the opening where the tray sits. Common sense tips that I forgot to follow... don't leave CD's in the changer for 5 years... especially in Florida. Don't put paper labels on CDs that you burn - one more thing to get stuck.