WD Passport not detected by my Macbook

5 minutes ago my WD Passport Hard Drive was working with my Macbook Pro (early 2011, OS X Yosemite 10.10.3) fine.

Suddenly, after disconnecting and reconnecting, my Macbook won't detect it.

I have tried:

- Using a USB cable for my iPhone in both USB ports on my Macbook to test the USB ports --- they work.

- Connecting my WD Passport in a different Mac --- it doesn't work.

- Using a USB cable from a different WD Passport with my WD Passport in both USB ports on my Macbook --- it doesn't work.

What can I do?