Sounds like the firmware update failed and the My Cloud is stuck. May want to contact WD Support directly (if you haven’t done so already) to see what they suggest.
If you had SSH enabled see if you can still access the My Cloud using SSH. At leas then you have some access to the device to try and figure out what is happening by using Linux commands.
Another option is to remove the My Cloud hard drive from it’s enclosure (will void the warrenty) and try pushing the firmware to the drive via one of the unbrick methods. There has been discussion elsewhere about pushing just the IMG files to the one or two partitions (depending on My Cloud firmware version) to try and fix a hung firmware update.
Not sure if it’s possible on the second gen v2.x My Cloud’s since they use a different Linux structure, but if one has a first gen v4.x My Cloud they can use SSH to trigger a firmware update from a manual file download. See the following post for more information.