That is NOT an uncommon occurance. DVD's and CD's use different mechanism's and circuitry on the drive so one can go out while the other continues to work. It's more common to have the CD fail than the DVD, but anything is possible. Try cleaning the drive with a lens cleaning disk and/or compressed air to see if that helps. Also try shutting down the system, unplugging the AC adapter, removing the battery, and holding down the power button for a minimum of 30 seconds to clear the system of residual charges. This often helps with this.
If neither of these two helps it's probably a defective drive that will need to be replaced.