"I have a western digital external hard drive and it has worked fine for 3 or 4 years."

 Well, 3 to 4 years is a typical lifetime for a disk drive.  Yes, I know, they all claim "100,000" hours or even more ridiculous numbers.

I hope you had a backup.

If you're lucky the problem is in the power supply of the external disk drive box-- they fail about as often as the disks.  You could try taking the drive out of the box and either trying a different power supply, or plug the drive into your computer's power supply, most have a few spare power connectors.

Does the drive sound like it is spinning when you put your ear to it?   Sometimes you can start up a stuck drive by carefully whirling it around its center of mass.  Just be careful to not yank out the power or data cable when you give it a good snap whirl.  If it starts up quickly copy your important files to a thumb drive or other disk as it may die completely very soon thereafter.

Or your connecting cable could be bad-- try a different USB cable.

You could try right-clicking on the disk drive, properties/check disk for ewrrors.  Although that almost never helps a whole lot.