Boot failure - nothing works

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http://askubuntu.com – Overnight, my ubuntu installation (latest LTS version) has stopped working. Apart from automatic updates, I never made any changes to the system. As of yesterday, the computer starts extremely slowly, eventually takes me to the GRUB screen, and after choosing the operating system, simply stops on a light purple screen. Recovery boot does not work. Does anyone have any idea how to fix this? Thanks (HowTos)