External Harddrive not recognized in My Computer from youtube by Rasmus Johnsen
featured video : DataBot Harddrive
featured video : PS4 Is Getting External Harddrive Support! - Adam Koralik
Showing you how to "refresh" your driver on your external hardrive
If your external hardrive doesn´'t even show up, then its most likely a hardware failure, meaning that your computer isn't the problem here, so you might get lucky by either:
1: Trying it on another computer
2: Buying a new cable for either data or even power.
If none of those has worked, it might be somthing wrong inside the hardrive, and if that's the case, i cant help you