Have you checked if the WD My Passport hard disk works on different ports on your computer? If the MAC has several USB ports, I recommend plugging the external drive in each of them because, for example, one of the USB ports may be failing or needing to update the drivers. Besides, I would like to know, have you connected the external hard drive on different computers with different operating systems recently?
If you haven't done it till now, I would like to suggest you try to access it with an Ubuntu Live USB. In the Ubuntu forum you could see the steps how make a bootable USB. Then you have to boot from it without installing the OS but just trying it on the MAC. Try to see if you could access the drive like that.
Can you hear the hard drive spinning while connected or everything is calm and does not feel any vibration when you touch the case? If you can't, the disk must be in a bad condition and you should take it to a data recovery company to extract the information.
Hope that I helped.
Western Digital representative