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)
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)