Copying my reply to a thread about not being able to launch GoG Galaxy.

"I had this problem and tried everything to get rid of it.
Finally, by accident, I entered the "performance enhancer" tab on CCleaner and noticed that GoG had ben set to "sleeping". Most other programs work despite that but it seems that GoG for some reason does not. I woke it up and had to manually add the games to the install list again, but since then it has worked as normal.
If nothing else has worked, check if GoG is in sleep mode."

Just adding it here in case someone has a similar problem and for the developers to see it and hopefully check it out. Other programs set to sleep mode wake up when launched but not this one for some reason.