I downloaded Nvidia driver from its official site and installed it in 12.04. But, I didn't notice that my laptop have an Intel HD 400 and a GT 620m video card, and Ubuntu does support the double video card. After installation, the Nvidia card did not work and Unity did not work well e.g. Unity 3D effects disappeared.
I have tried to uninstall the Nvidia driver but my desktop is missing, there was only a wallpaper. Here's how I installed the Nvidia driver:
stop lightdm sudo sh xxx.run start lightdm
Now I want to restore this to the first place. I'd be grateful for any advice.