Is the burner your only optical (CD) drive?
Are C, D and E your hard drives?
Are you running Win XP?
If so, restart the computer in the Safe Mode.. (hit the F8 repeatedly after you first turn the system on to get to the Startup Menu...then choose Safe Mode)
Is the CD drive there in the Safe Mode?
If not, the drive could have failed. They do that sometimes.
Put a disk in the tray and note where the label is facing...close the tray and listen for it spinning. After a minute or so, open the tray. If you didn't hear the spin up and the label is still in the same orientation... there is a pretty good chance the drive has failed.