1. While on the login screen:
Press Ctrl + Alt + F1
or Ctrl + Alt + F2
(Virtual instance)
2. Login to the shell prompt with username and password
3. Type the following commands:
dpkg --configure -a
(fix any corrupted packages)
sudo apt-get update
(update package repository)
sudo apt-get upgrade
(upgrade the package repository)
sudo reboot
(reboot the system)