From the menu bar, select
▹ System Preferences ▹ Energy Saver ▹ Power Adapter
and uncheck the box labeled Put the hard disk(s) to sleep when possible, if it's checked.
If the drive has more than one interface (USB, FireWire, Thunderbolt, eSATA), try one of the other interfaces.
Check that the data cable is securely inserted at both ends.
Try a different cable.
If you're connecting the drive through a hub, connect it directly to a built-in port on the Mac.
If you're connecting it directly, try a different port.
Disconnect all other devices on the bus, or as many as possible.
Test the drive with another Mac. Test another drive with this Mac.
If the drive is bus-powered, but has an AC adapter, connect the adapter.
Start up in Recovery mode and launch Disk Utility. Is the drive recognized?
Start up in Safe Mode and test.
Reset the NVRAM.
Reset the System Management Controller.
If the drive doesn't work under any of the above conditions, and if another drive does work with the same Mac, then the drive has failed. You may be able to salvage the mechanism by removing it from the enclosure and installing it in another one, or in a drive dock.
