None of my WD drives (My Book or Duo) are being recognised

From this morning none of my WD external hard drives (2 x My Book, 1 x My Book Duo) are being recognised by my PC running Windows 7. Other non-WD external hard drives all recognised. Any suggestions as to how to solve this gratefully received!

What have you tried so far? Do they work if they are connected one at a time (Independently)?