Comment 11 for bug 1589006

If now you're able to log-in but there's no frame, it usually means it's a compiz problem :D
Back with Ubuntu 10.04-12.04, when I played very frequently with Compiz settings, I messed up the system many times, and the solution has always been to purge everything related to compiz/unity (even the settings in the home directory, with rm -rf ~/.compiz-1 ~/.config/compiz-1 ) and then reinstalling it (maybe with the ubuntu-desktop metapackage).

Also, make sure nvidia's drivers has been correctly installed and they has been loaded (look at the output of lshw in the display section)