First keep in mind this is a very old system! So anything you do might break something as the plastics get brittle!
From the sounds of it I would say the HD cable or its connections to the drive are in trouble. What is not very obvious is the drive has two connections on the PATA connector. One for the data ,and the other side for the power. Carefully lifting the drive up you should be able to see if power is even getting to the drive, use a DVM and measure the connection points. If you have power then you either have a bad data side connection of the cable or the drive its self got damaged.