You might remember that, by default, you had six virtual consoles that supported command line login sessions. It's not obvious but, when you start an X session, that X session runs on virtual console seven. If, at any time, you want to switch from X back to one of those earlier virtual consoles, just use the key sequence Ctrl+Alt+Fn, where n is the number of the destination console.
Once you're done with that virtual console, you can always switch back to X with Alt+F7.
Note that, to move around the command-line virtual consoles, or to move from one of those virtual consoles to your X session, you need type only Alt+Fn. However, because that key sequence is most likely already mapped in an X session, to move from the X session, you need the sequence Ctrl+Alt+Fn.
In fact, you can, if you want, use the longer sequence on the regular virtual consoles as well, but it's not strictly necessary.
The above tip also shows you the value of having started X in the background with startx &. If you did that, you can switch back to the console from which you ran that command and continue to work, since the X session is running in the background.
Otherwise, while you could still switch to that console, it will be tied up running the X session.