My My Book Essential stopped working suddenly

I was taking backup of my work a couple of day back and all on a sudden, the unit stopped working, the power indication gone and my system stopped detecting my “My Book Essential”.  What can be problem?  There was no power fluctuation, the pc is connect from a UPS and the PC works well except my “My Book Essential” unit. I tried restarting the system and started connecting the external unit all over again.  But that did not work.  What is the solution?