Does DVD drive appear in Device Manager ?
is there an error message ?
can you boot up from a CD or DVD eg Windows disk, bootable Antivirus disk etc (to eliminate hardware fault)
corrupt upper or lower filter drivers can cause problems seeing DVD drives
Microsoft has a fixit for this problem (use Google)
ensure you only download fixit from microsoft.com (for safety)
AND for correct version of Windows (XP Vista or 7)