What color is the LED on the front of the My Cloud, provided you haven’t already removed the internal hard drive from its enclosure?
If that LED is blue then it should be accessable by the Mac Finder or via a web browser. If white then try a 4 second or 40 second reset to see if the problem clears. See the following WD Support document for more in formation on how to reset the My Cloud: http://support.wdc.com/KnowledgeBase/answer.aspx?ID=10432 If the LED is red that typically means something more serious has happened to the drive.
Edit to add: As was previously indicated the Apple El Capitan update is causing several problems with user’s ability to access and use their My Cloud devices. There are numerous other threads discussing the various issues with El Capitan. Here is one of the many threads:
