1st Way to solve your issue:

Try to reconfigure lightdm (if it doesn't solve your issue then go for Alternate way mentioned below.)

• Open terminal(CTRL+ALT+T) and execute following command:

  
  
  

  sudo dpkg-reconfigre lightdm
  
  




• Then chose lightdm from the list. As shown below:

_{(source: akamaihd.net)}

• Restart your system:

  
  
  

  sudo reboot now
  
  

It should fix your issue.

an alternate way, which would solve your issue. Give it a try.

Installing a new display manager can fix your problem. Steps to install GDM(a display manager much like lightdm):

• Open terminal(CTRL+ALT+T) and execute following commands:

  
  
  

  sudo apt-get install gdm
  
  sudo dpkg-reconfigure lightdm
  
  

  
  
  

  then chose gdm from the list:

_{(source: akamaihd.net)}

    sudo reboot

• If it's still not fixed, try removing lightdm after installing gdm:

  
  
  

  sudo apt-get remove --purge lightdm
  
  sudo reboot
  
  

Reply if something goes wrong..