Not able to login via GUI : Error in locking Xauthority file

1. Switch to command line "Ctrl" + "Alt" + "F6″

2. enter login, provide user name and password

3. type startx

   it should switch to GUI mode

but if you get Error in locking Xauthority file, try following:

The error i got means i do not have authorize to access $HOME/.Xauthority file in my user directory.

If you have the same problem, simply run chown as root to change the ownership back to the user. At the terminal as root, go to /home/user
# cd /home/user

list all files with ownership
# ls -a -lh

change file/files ownership to your user
# chown user:user .Xau*

reboot the machine, everything should be working as normal now.

Source: http://namhuy.net/1077/fixing-error-in-locking-authority-file-xauthority.html

Grub Re-installation

You need to have Ubuntu Live CD or Live USB. Normal session can be used to repair the grub. Boot using your Ubuntu Live CD or Live USB, while booting choose Try Ubuntu.

Once booted then open a terminal, and run the following command one by one to install the boot repair.

To add boot-repair to the repository

sudo add-apt-repository ppa:yannubuntu/boot-repair

To Update your repository

    sudo apt-get update

To install boot-repair

sudo apt-get install -y boot-repair

Once Installation complete run boot-repair on terminal by typing the following command or select it by System->Aministration->Boot Repair.

boot-repair

NOTE: Update the Boot Repair if its newer version is available.

Source: http://www.howopensource.com/2012/05/reinstall-recover-grub-from-ubuntu-12-04-live-cd-usb/