Check if the saves are present in the save game folder. It's located in "\Documents\The Witcher 3\gamesaves".

All the save games were there.

I ran a repair/verify from the game menu and it's fixed it. Save games are there and cloud syncing is back..

Was twitching a bit for a while.

Thanks for your help Sarafan

Hmm this just happened to me, played fine yesterday.
Fired up for a quick game today - got a brief message there was an "update" (I was already on the latest version, this update is very old (1.31 ) and now all my saves have disappeared back to around Feb 2018.
(is this when GOG cloud saves started?)

I am guessing something has gone wrong with GOG cloud saves?

Tried repairing and not fixed.
I was just enjoying my second run through of the game...

If you're running Win10 you might be affected by the botched october update which deletes files from your Documents directory.

Not as far as I can tell, not had any windows updates for a couple of weeks etc.

I am convinced GOG cloud has caused this, maybe an issue with t the cloud? Or maybe my GOG galaxy not signing in properly maybe?
Saved games that remain on my PC stop at Feb 2018 (I've been playing Witcher 3 regularly since around OCt 2017 to date).

Something weird has happened to lose all progress since feb 2018..!