It could be a sign that one of the cables to the drive has become disconnected, or that the drive has failed, the latter is more likely.
To confirm, test the drive with the manufacturer's CD-based diagnostic software which you can download via one of these links:
http://www.tacktech.com/display.cfm?ttid=287
You can create a CD from the downloaded ISO file using IMGBurn:
http://www.filehippo.com/download_imgburn/
Boot your faulty computer from that CD.