When you delete a file on a locally-attached drive, it goes into the recycle bin, which gives you a chance to undo the damage. However, when you delete a file from a network drive, it deletes it immediately. There’s no trash can involved, it’s just gone.
If you want to try getting your files back, it is crucial that you stop adding data to the My Cloud immediately. You don’t want to risk overwriting the files you’re trying to recover.
Perhaps others have done this and can advise you better, but the only thing I can think of is to install onto the My Cloud a Linux command-line tool for undeleting files. Something like
I’m pretty sure doing this voids your warranty, though.