I was browsing the web with Firefox on Ubuntu when suddenly the browser became unresponsive, it's icon on the taskbar became a grey question mark, then the window closed. I tried to open firefox again by searching for it except it didn't appear; when I type firefox into the terminal I am presented with command not found error. What could possible cause a program to disappear from the system like that?
I noticed that Ubuntu software center seems to have disappeared also.
I am running 12.04 lts and was not then installing anything, and just browsing a webpage.