Aside from making sure that X11.app is running, check that you can run another X program from a Terminal window (say xterm and/or xhost). Some things can confuzzle the xauth authentication, such as disappearing network connections (disconnecting from a WiFi network, or disconnecting a modem connection).

In reply to Re: Tk OSX couldn't connect to display :0 by Fletch
in thread Tk OSX couldn't connect to display :0 by Washizu

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