Do you see your HDD in the Finder sidebar? You can also check Disk Utility (located in Applications> Utilities folder). If you see the HDD greyed out it means that it has failed and it needs to be replaced. You can also check if your USB cable is in good condition. I had an issue where one of my external HDDs did not showed on my Mac and PC, the issue ended being a damaged USB cable.
Hope this information helps!