Have you made sure that "External Disks" are checked in the Finder's Sidebar and/or General Preferences/Settings?
What file system is on the WD external drive?
Have you tried to click the "Mount" button in Disk Utility?
Have you tried running First Aid on the external drive?
Have you tried connecting the drive to another USB port? Using another USB cable?
Disconnect all other external devices. Connect the WD drive directly to the computer.
Try rebooting the computer...a simple reboot can work magic.
If you are not using any third party proprietary software to manage this external drive, then try booting into Safe Mode to see if you can access the external drive.
What is the exact model of the Mac and version of macOS on it? You can get this information by clicking the Apple menu and selecting "About This Mac".
Can you access the data on the drive using another computer?
It is possible this hard drive is failing. You can try to check the health of this drive by running DriveDx (free trial period). This will require you to install a special USB driver in order to attempt to access the external drive's health information, but even with this special driver....some external drives may not allow the necessary communication. Post the complete DriveDx text report here using the "Additional Text" icon which looks like a piece of paper on the forum editing toolbar.
FYI, you should always have a backup of all external media (including the cloud) which contains important & unique data.