This is in the known issues list at the top of this forum:
When that happens, you can force-quit the game (Alt+F4 in Windows) and restart it to return to the game in progress. There is a small chance that it won’t work, as reconnecting to single-player adventure games is a new feature, but worth a shot.
For future references, if you want to link screenshots, use the Preformatted text button </> to put single back quotes around the text `like this`.