Look for the CD/DVD in Device Manager. Can you still find it listed in Device Manager?
If not, then Windows can't detect it probably meaning bad cable, connection or the drive itself has failed.
If yes, then you might have a problem with "filters". See [post=815753]CD/DVD or Disk Problems? How to Fix Problems Caused by Filters[/post]
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Tho... even if not found in device manager, is still worth giving trying to fix filters a shot