how do you know its jammed? i mean does it spin up?? somtimes some failed discs refuse to eject, i usually restart and hold the mouse key to force eject them.
If it is not physically jammed, you don't have to reboot:
Open iTunes
Choose Controls: Eject Disk (Command-E)
This ejects the mechanism whether or not the Mac 'thinks' there is a CD in it or not.
If the disk is actually jammed, sometimes you can slide in some thin card stock and "guide" the CD out.
If you are dealing with a miniCD or a business card CD that you have inserted into a slot loading mechanism ... then my condolences. Take it to a technician.