If you have a large media collection on the My Cloud it may take the Twonky Media Server some time to rescan or rebuild the media database.
It is recommended NOT to just unplug power from the My Cloud, rather to perform a Shutdown through the My Cloud Dashboard, or issue the shutdown command through SSH before unplugging power on the My Cloud. Some of us (myself included) have experienced problems (in my case some media files not streaming properly) when simply pulling the power from the device and not performing a shutdown. Now I perform a shutdown via SSH when not using the My Cloud device for long periods of time and no longer have those problems.
Edit to add: There is some discussion on how to use SSH to issue the shutdown command at the following thread.
https://community.wd.com/t/disconnect-the-power-outlet-for-storm/96600
