Ubuntu freezes before login screen

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http://askubuntu.com – Recently I've had this issue where Ubuntu freezes before I can get to the login screen or even the boot animation. It started off with myself leaving the PC on overnight, steam downloading tf2. I come down the next morning to see that Linux needs me to login again. I do so, but it seems like steam is closed. I go to system settings to test if my displays are able to run together again, I also click on steam, and Firefox. At that point, the os freezes. My mouse is still able to move about, but the system won't respond to anything that I do, mouse or keyboard. After giving it time to recover, I (HowTos)