Is this relevant? If not, it might be helpful if you provide some system info.

I had a simmilar problem, a reboot fixed it. Then again, I barely ever shut down my PC and leave it on sleep a lot, so my system being unstable might have been more of a problem than the game.

Still, never too leate to try the old turn it off and on again!

this is in google search so I will post it here.
I have this issue today.
seems like it just cause by a steam update.
close the steam and restart it so it can install steam update.
after that it works again.
I think it's because game need to sync to steam cloud.
but they block you out when there is an update and you haven't install it.