How to fix "ubuntu is running in low-graphics mode" ?

I solved this problem by reinstalling ubuntu-desktop.  

Open Ubuntu in console mode.
Give username and password.

or

When the message that "your system is running in low-graphics mode" appears, press Ctrl+Alt+F1, then login with your credentials.

And then, run the following commands:

• sudo apt-get install --reinstall ubuntu-desktop
• sudo reboot

It is tested on Ubuntu 10.04 on date 09/05/2013