Mine did the same a few months back - i.e. no lights fan only apparently no disk activity - but came back to life after following the full reset process as supplied to me by WD support - I skipped the first step as I had disconnected the power already. As I recall, It didn't come back to life immediately - I left it connected and came back a few hours later and it was back:
Thank you for contacting Western Digital Customer Service and Support.
I recommend you reset your drive via the following process. Once you have done that, go into the Anywhere Access, and under File you should be able to scan for devices.
1. Press and HOLD the drive's power button until you can no longer hear the fans spinning.
2. Disconnect the power cable from the back of the drive.
3. Press and HOLD the reset button on the back of the drive 10 seconds. See the link below for pictures but do NOT release the reset button when proceeding to step #4.
Answer Title: How to reset the username, password, and network settings of a WD My Book World Edition hard drive.
Answer Link: http://wdc.custhelp.com/cgi-bin/wdc.cfg/php/enduser/std_adp.php?p_faqid=1521&p_created=1172595028
4. Reconnect the drive's power cable while continuing hold the reset switch. Do NOT release the reset button when proceeding to step #5.
5. If the drive does not turn on by itself, press and HOLD the drive's power button until you see the light rings illuminate and spin around on the front of the drive.
6. Release the reset button