Ubuntu Unity randomly crashes, and then the command 'unity' isn't recognized

For the last bit, unity (and perhaps other things?) would crash after something like 20-30 minutes of use. First my launchbar would disappear, all of the upper right hand icons disappear and then all my programs freeze. If I press my power button, the login/shutdown/restart screen shows up but all the text is squares. Then the entire GUI freezes. If I manage to open up a terminal before everything stops and I type in 'unity', it says that the command isn't recognised. Any suggestions or help would be greatly appreciated. Thanks!