What you need:
1. A stuck cd/dvd drive
2. A paper clip
First, turn off your computer before doing this!!
Step 1: The Paper Clip
Step 2: The Small Hole
Step 3: Stick the Clip Into the Hole
Step 4: Finish
To prevent this in the future keep your CD's clean and free of finger prints and scratches using some kind of CD paper sleeve or a jewel case properly stored in a CD rack. And make sure your discs aren't warped, because this may cause problems as well.