Ubuntu, like most other distros, prompts you to install system updates. You can control the schedule using the Update Manager and set it for daily, every two days, per week or every two weeks. I prefer the weekly updates. There are other options as well but I will not be highlighting them at this time. Typically the defaults on most options will work for most users but they are fairly straightforward and do not need much explanation. Most of the time updates will install just fine. However once in a while the internet or your computer just doesn't cooperate. The update then gets hung and maybe the system will not even boot fully. It is times like this that a little knowledge of the command line will be very beneficial. If possible boot to the prompt and issue these commands:
sudo apt-get update
apt-get upgrade -u
This will perform the rest of the update just as you were using the gui's Update Manager. It will be important to note that wireless users may or may not be connected, so it might be wise to connect via a cable if possible. Running the unfinished upgrade this way just may prevent a needless re-install of the OS. But you have /Home on a separate partition anyway, right?