Has anyone else experienced this? Fairly frequently I find that kwin on F25 is not starting on login, there are no window decorations (e.g. title bar) and all new windows start at top left with no apparent way to move them. The solution seems to be to start a console and run "kstart kwin". This morning however I didn't even have a desktop, just a cursor and an otherwise black screen. What was working was that typing brought up the command bar at the top of the screen, so I was able to start konsole. "kstart kwin" and "kstart kwin --replace" didn't seem to achieve anything (though maybe I could have waited longer), eventually tried "kstart plasmashell" and things seemed to run again.

This is the nouveau driver. I've had problems on F25 with getting stuck at the splash screen ("K") after login, but adding nomodeset to the kernel line in grub seemed to alleviate that, and I was using nomodeset this morning.