My cotputer's dvd drive does not read cds but only dvds what cld be the problem?

SOURCE: not reading DVD

It's common problem with combo drives in notebooks, they don't seem to last long. Try this:- Go to my computer>properties>hardware>device manager and look down the list for your cd/dvd drives and click the '+' sign then highlight your dvd drive and right click to delete. Restart the pc and windows will find the drive again automatically and install the drivers. If the problem persists the laser is faulty in the drive and the drive will need to be replaced.

SOURCE: DVD driver and handycam dvds

None at all - the DVD player has a chipset that is incompatible with that type of disk.
Check the player manual for the disk types it can handle

SOURCE: PIONEER DVD-RW DVR-112D problem

I've just come across this notice from Pioneer. Sounds like you might be having the problem that they are describing in the faulty drives.

SOURCE: Same problem: Optiarc won't read a DVD, but CDs are fine

Clean the lens using a cotton buds using a dish washing liquid(dont wet it too much).Then rinse it with tap water and let it dry for a couple of minutes.

SOURCE: the LG 18x dvd writer does not read all dvds but

if it is comes under warrenty