The Xbox One's instant resume feature has been reported to cause some corruption in The Witcher 3's save files.
The issue can be manifested in a couple of different ways. Some are reporting that upon turning the console back on and resuming the game, they get the message "you cannot save right now." For others, the game gets stuck on the loading screen and would lose a few hours worth of progress when it eventually works after restarting.
If that happens to you after turning the console back on, restart the game and see if you've lost any progress. If you did, delete local save files and download the ones in the cloud, provided they're older. Otherwise, restarting the game should automatically load the most the recent save.
Until this issue has been resolved, it would probably be best to save and close the game manually after every session.