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In order to figure out the problem with a non-working Mac hard drive, you'll have to remove the drive from your Mac, plug it into a USB adapter, and plug the adapter into a working Mac computer.  Most Mac hard drives will use SATA connections, so you'll need a SATA to USB or USB-C adapter. It's a menu item at the top of the screen. A drop-down menu will appear.  If you don't see this menu item, first click the desktop or open Finder. You'll find this in the Go drop-down menu. The Utilities folder will open. Double-click the Disk Utility app, which resembles a grey hard drive with a stethoscope on it. Click the name of the attached hard drive in the left-hand sidebar to do so.  If the hard drive doesn't appear here, it may be too damaged to read at all. You should take it into a repair service to have it checked for recovery options. It's a tab near the top of the window. You'll see this prompt appear at the top of the window. Once Disk Utility finishes scanning the drive, it will provide you with a list of drive issues, at least one of which may be responsible for the drive's failure.  You will most likely receive a message saying that the hard drive is about to fail. If possible, back up the hard drive—you cannot repair it from this point. If your hard drive shows more than a couple of significant errors, chances are that it needs to be repaired physically. You'll need a tech professional to repair your hard drive for you.  In most cases, the repair service will only be able to recover data off of the hard drive; you most likely won't be able to use the hard drive again.
Attach the non-working hard drive to a working Mac computer. Click Go. Click Utilities. Open Disk Utility. Select the attached hard drive. Click First Aid. Click Run when prompted. Review the results. Take your hard drive into a professional repair service.