Failure to recognize a hard drive can be caused by a jumper pin missing, a bad hard drive EIDE or power cable (they get brittle and disconnect as they age).
But usually, it is the failure of the hard drive. They often fail for no good reason, just like a light bulb burning out... or a chip failure on the back of the drive. Sometimes the magnetic material on the internal plates will begin to curl up and flake off. Others will have the bearings burn out or freeze up preventing the plates from turning. Still others have glass plates covered with magnetic coating that suddenly shatter from the centrifical force.
Try borrowing another drive to try, just to test the connectors.
It can be tricky to save the data on that drive, but you can expect to be successful about 66 to 75 percent of the time.