DVD/CD combo drives use different lasers for each disk type. I've seen MANY combo drives over the years where the DVD reading sub-system fails, yet the CD reading sub-system still works, it's the most usual failure reason for DVD combo drives (in my experience).
If the usual remedies (like cleaning the drive of dust/debris, checking the disks for damage, etc.) don't help, then the fix is to replace the DVD combo drive; which is a pretty cheap/easy repair in most scenarios.