What should I do if I can't reach MyBook on my WD External Hard drive using iMac?

It could just be the USB cable or the WD drive enclosure that has failed.

Sadly, if you do not have a second redundant backup of this data on another drive?

Then you need to take or send the WD drive to a professional data recover service.

If the computer was backing up to a Time Machine External Hard Disk all of your data is safe. This is why you have a Time Machine backup to safeguard your data. If you never setup Time Machine then yes, you are learning a tough lesson.

Finally, if you are not familiar with Time Machine (it's been in Mac OS about 15-20 years) please refer to Back up your Mac with Time Machine. The External Hard Disk I would recommend rather than the cheap Walmart WD enclosures is the OWC Mercury Elite Pro due to its high reliability, reasonable price and the service provided by the vendor.

NEVER EVER operate any computer, phone, iPad etc without backup!!!!!!!!!!!!!!!!!!!!!!

Disconnect all other external devices in case one or more of them is causing a problem with the WD drive.

Connect the drive directly to the Mac & try using the other USB-C ports especially on the other side if using a laptop.

Some drives may need to be connected to their own power adapter or a powered USB3 hub since the computer may not always be able to provide enough power to some drives.

Some third party software can cause problems with external devices as well. You can try booting into Safe Mode to see if the drive is visible (not an option if you are using the manufacturer's proprietary software to manage the drive including third party NTFS drivers).

And like @Tesserax mentions, you need to distinguish between the drive not mounting and not being seen physically which are two completely different things with the latter being the worst case.

There are a lot of reasons for a drive not being "seen" on a Mac as you can see from several of our posts here.

Have you even tried a simple restart of the computer? Many times that can fix an odd issue.

Any and all external and internal storage drives can fail without warning at any time.

Try disconnecting the WD from your Mac, restart your Mac and then reconnect the WD.

What are/were you using the WD MyBook external drive for?

If it was your Time Machine backup, then replace the drive and start new backups.

If it was files to free up space on your Mac, then they could be lost if you did not have a second backup.

It is very possible with that drive model for the electronics in the enclosure to be at fault, not the hard drive inside the enclosure. WD drives are great but I cannot say the same for their outside enclosures.

If the others' excellent suggestions do not work, remove the hard drive from the enclosure and use a bare drive adapter to connect the bare drive to a USB port on your Mac. Yes, you can open the enclosure—I had to to it with my MyBook enclosure when it acted up. Web search for "wd mybook takeapart" and you will learn how. I used a YouTube video to learn how to open mine without breaking the case.

With the drive out of the enclosure and fitted with an adapter, you can see if the bare drive is viable.

You're welcome. We're hoping that your precious memories are not lost.

If you must store your pictures on an external drive, always make a second backup on a second external drive to protect your precious memories.

When this drive is connected to your Mac, does it appear in the Disk Utility?

... or in System Information > Hardware > Storage?

You're very welcome ... but, did you check in either of these locations to see if your WD drive shows up?