Did you run or were you running a Safepoint / Backup from the My Cloud Dashboard which would backup the My Cloud to a USB hard drive attached to the USB port on the back of the My Cloud?
Often, unless there is a specific method for unlocking ransomware encrypted files one is typically out of luck if they don’t have an unaffected backup of the ransomware encrypted files/data.
Chances are the My Cloud was affected because the ransomware program was downloaded and run on a local network PC. Once on that PC it may hide itself so malware/security/antivirus programs will not find it. Often one has to scan the PC using a emergency boot disc/usb flash drive that some security or malware or antivirus software companies provide (some may do so for free).
The problem is, as always, once infected there is no way to know for sure if you have totally eradicated the ransomware (offending software) using various antivirus/malware/security software. Best option, while not optimal, is to completely wipe the hard drive and restore the data to it from clean backup media or backup locations. Not having a backup of the backup presents problems.