Sounds like the DVD laser / head broke. To the best of my knowledge (and I may be wrong), there are actually two different systems in a combo drive, one for DVDs and one for CDs. If that's actually the case, I can see why one could break while the other still works.
Is the notebook still in warranty? If not, you would have to either get the same part (notebook drives are not universal as desktop drives are) or settle for attaching an external DVD drive.