New WD My Book Essential 3 TB No Longer Spinning Up

WD External Drives External Drives for Mac
4

When all is shut down, including the Mac, try -

  1. unplugging your Mac and the HDD from the mains,

  2. count to 30

  3. plug everything back in again and restart.  

Does it work okay now?

Or

Did you look for a button craftily hidden button on the rear of the HDD?  If not, and there is indeed a button at the back, press it once.  Did that fix it?