Do a search for “unbrick”. There are generally links to img files in those threads that one uses to unbrick their My Cloud.
If you are replacing the hard drive, which I’ve done several times now, you don’t need the same image file for the 3T that was in your My Cloud enclosure. Just about any image file, so long as its from the same generation version (older v3.x/v4.x or the newer v2.x) as your My Cloud, can be used and the My Cloud will reset the size once one performs a quick system restore after the unbricking process.
I ran into problems with most other unbricking procedures when replacing the hard drive. Including being unable to access the Dashboard post v4.x firmware upgrade. I detail my experiences and what ultimately worked in the following thread/post.
While the steps I outline in that post worked for me they may not work for others.