If you were in Xwindows/Gnome/KDE/whatever and you pressed CTRL-ALT-F1 through F6, you've merely switched to one of your consoles. Xwindows is still running. To switch back to it, just press ALT-F7 from one of your console windows. No need to restart Xwindows or the computer.
If you've exitted Xwindows completely, you'll need to run 'startx'...