The obvious thing is a cable check and plugs etc but I bet you've done this. Other things to consider;
Do a system/hardware device check. Any ?
Open Bios and check the drive entries.
Try another cable.
If you're master and slave...swap over then try just the faulty as Master.
Think carefully..what were you doing PCwise just before the failure ?
Take the unit out and get someone else to try it.
All the best.