Cd ROM’s not showing in My Computer

Hi,
I have a client with W XP Home. This has happened a couple of times. When he gets a CD R from one of his workers and he goes into my computer the CD RW and CD ROM are not there. Last time all I did was a reboot and it appeared. No cameras or other devices have been aded to the system. One other time on a different clients computer the same thing happened and when I unplugged the camera the cd’s appeared.
Thanks