When you open it, you will see 3 things:
The drawer mechanism, the part that holds/spins the CD, and a part that moves into place to read the CD. This part has the lens.
Try loading a CD with the top open to make sure nothing is being obstructed. If everything is moving fine, then open the drawer with the top open.
With a clean Q-Tip, clean any dust from the lens - it's pretty obvious which part is the lens.
If there is other dust inside, clean it all out.
If your CD is a walkman or doesn't have a tray, then it's not worth fixing. Just chuck it in the garbage.