Xlib: connection to ":0.0" refused by server?

If I'm logged in as 'bob' and I su root, then try to start some thing like 
emacs I get this:


root@bob cups # emacs error_log&
[1] 6860
root@bob cups # Xlib: connection to ":0.0" refused by server
Xlib: No protocol specified

emacs: Cannot connect to X server :0.0.
Check the DISPLAY environment variable or use `-d'.
Also use the `xhost' program to verify that it is set to permit
connections from your machine. [This is the same machine!?]

This just started and I don't know what Ichangedd to cause it.
How do I make it go away so that it works again?

If I'm logged in as root I can start things from the command line just fine.