Welcome to Tom's Hardware, @KG_01!
This sounds pretty unfortunate.
If you have unplugged the external HDD while it was still active (reading/writing) on your Mac, you could have damaged its file system or even worse - its hardware. My recommendation is to check how the HDD will get recognized if you use a different USB cable and try it on another Mac computer. I'd also suggest you go to Disk Utility on your Mac and
test the external hard drive for defects from there.
If nothing helps, you could try using third-party data recovery tools at your own risk. But since we cannot determine the exact damage, I believe a professional data recovery company would be your best bet on getting any of the files from the external HDD back.
Hope this helps you, though. Good luck!
SuperSoph_WD