I'm new to Ubuntu and I recently installed it on my PC. I logged in immediately after installing and it was working okay. I was told (in a beginners guide to Ubuntu) to update packages, so I opened the command line and typed "sudo apt-get update." Soon after, while I was messing around a bit, the screen froze (everything stopped responding, except for the mouse pointer). I force shut-down the computer, restarted, and tried to log on again. This time, before I even got to the login page, the computer displayed only a purple screen and wouldn't respond to anything. This happened two more times and I can't figure out how to fix it. Any suggestions?
EDIT: I forgot to mention it's version 12.10.