This is like any othe PC DVD drive, sometimes it get stuck for what ever reason. If rebooting the iDrive did not work for you you may have to do a hard reboot, which means removing power. try disconnecting the battery, wait a few mintues and then reconnect, then try the eject button. If this don't work then you have a mechnical issue which may require some drastic steps such as suggested by jamie645ci if you do not want to go to the dealer.
2008 650 Convertible- 6 Speed manual- Sport package- HUD- Lane Departure- Comfort Access- Merino Leather- HD Radio- SAT