Find a large paper clip, then unbend it until you have a straight wire about four to six inches long.
Then look for a tiny hole in the front bezel, usually just below the CD tray door.
Stick the paper clip wire straight into the hole, firnly. If done with enough force, the tray should open and you can remove the disc.
Now try another disc. If this happens again, your drive is likely worn out. If it is a CD-RW DVD drive, replacements are $29 at Wal-Mart, Best Buy, and others. If it is a DVD-RW, they are usually about $49.
Avoid getting the Sony, HP, LaCie, and HST drives. Usually the LG and Samsung are the best for the price. A Plextor is often a better drive, but at a very high price.