DVD drives tend to die and become unable to read DVD's (CD's still work) when they become very dusty or dirty inside. I have seen the same thing happen to several drives. There is really nothing you can do other than replace the drive. You can try one of those lens cleaning discs that they sell, but I don't know if they really work.
Still, its probably just better to install a Superdrive, as you can pick them up as cheaply as $50 these days.