Control + s in Xterm
If you happen to be a noob like me, and are used to using a fancy graphical IDE like eclipse, NetBeans, or (God forbid) Visual Studio, then you're probably used to hitting <control>+<s> to save something (I think emacs uses "control + s" for saving too). If this is the case, then you've no doubt hit "control+s" by reflex when you wanted to save something in vim or vi. If you're using GNOME's Terminal or KDE's Konsole this shouldn't be a problem, although nothing will actually be saved.
But in xterm, "control + s" makes it so that you can no longer see terminal output. Basically, whatever you type is still processed by the shell, but you just don't see anything (I can't figure why anyone would actually want to do this). So for the uneducated, it looks like xterm has locked up and you've lost all your precious code. Fear not my friend! You can exit out of this funky mode by hitting "control + q."
In summation, if you ever hit "control + s" while using xterm, and everything appears to lock up, hit "control + q" to return to normal.