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[ubuntu] stuck on command line can't reinstall ubuntu

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http://ubuntuforums.org – i brought a brand new laptop wiped windows 8 off and installed ubuntu 64bit 12.10 unity, (latest) then i wanted to swap the desktop for an earlier version i did "sudo apt-get install gnome-session-fallback" but then when i rebooted the machine it just opened to command line so then i did sudo apt-get update and sudo apt-get install -f but its not working so then i just put ubuntu back onto the pendrive and plugged it in as i hoped the computer would just reboot from the pendrive again, but that also didnt work and all i get is a command line i really dont know much about computers a (Hardware)