Save files are not visible

The save files are unfortunately unrecoverable and can't be loaded anymore. Microsoft OneDrive syncing is likely interfering with the game. You can try excluding the OneDrive\Документы\Klei\OxygenNotIncluded folder from OneDrive or disable OneDrive altogether and see if that solves the issue on future saves.

Does that mean that i have to start the new game every time?

Is there any opportunity to continue playing?

Are the save files lost?

It shouldn't keep happening, especially if OneDrive is off. It's possible a Windows update re-enabled OneDrive, do you recall updating your computer while playing on this save?

Could it be related to putting your laptop into sleep mode after playing before Steam had a chance to sync the save? 

If you're worried about it happening again I would suggest starting a local save (or moving a current cloud save) and double checking OneDrive is disabled.