If the Viera TV uses DLNA to access the My Cloud then check the My Cloud Dashboard to ensure Media Serving is enabled on the Shares containing your media files.
Sometimes a My Cloud restart may work to fix certain issue. Also it helps to set a static IP address fro the My Cloud, either within the router’s DHCP server administration page or in the My Cloud Dashboard > Settings > Network section.
The following thread may also be relevant because your using the media server.
https://community.wd.com/t/faq-twonky-dlna-media-server-setup-use/95373
