First, you need to know how the optical drive works. Basically, there is a different laser for each purpose. For example, there is 1 laser for CD read and another for CD write. When the optical drive is used continuously, some lasers might get misaligned. The result is that you will lose that function. I believe your DVD read laser has become misaligned, so if you want to read DVD's, you have 2 options:
1. Replace the drive.
2. Buy an external drive. They can be found for about $50 at any electronics store. Most of them can even write DVD's!
Hope this helps!