Ubuntu running in low-graphic mode

Solution 1:
Reconfigure xserver xorg, with the command

sudo dpkg-reconfigure xserver-xorg

Solution 2: (Try only if you know what you are doing)

• Take a backup of xorg.conf file:

sudo cp /etc/X11/xorg.conf /etc/X11/xorg.conf_bkp

• Open the xorg.conf file for editing:

sudo cp /etc/X11/xorg.conf /etc/X11/xorg.conf_bkp

• Delete this Section Device:

Section “Device”

Identifier     “Device0″

Driver         “nvidia”

VendorName     “NVIDIA Corporation”

BoardName      “GeForce 8400M GS”

EndSection

• Edit the Section Screen as:

Section “Screen”

Identifier     “Screen0″

Device         “Device0″

Monitor        “Monitor0″

DefaultDepth    24

SubSection     “Display”

Depth       24

Modes “1440×990″

EndSubSection

EndSection