The My Cloud was not reformatted with “Twonky”. Twonky is a Media Server that is part of the My Cloud operating system.
To start you want to access the My Cloud Dashboard. This is the page where you can configure the My Cloud and its various options/settings/features.
How to access the Dashboard on a My Cloud (single bay) device
From the My Cloud Dashboard you might be able to see if it is still listing data on the drive in Shares or in the Capacity section.
If the data is really gone from the drive one can restore their data from the Safepoint / Backup option IF one had that feature enabled previously and was backing up their My Cloud data to a USB hard drive attached to the My Cloud.
While rare, updates have been known to cause issues. One can use the forum search feature, magnifying glass icon upper right, to search for past discussions on similar issues. For example one past discussion:
If using Windows 10 and the My Cloud isn’t showing up properly in Windows File Explorer. Check that SMB1.0/CIFS is enabled, Microsoft disables this feature in recent Windows 10 updates.
How to Enable SMB 1.0/CIFS File Sharing Support on Windows 10
