I cannot find a way around this, maybe you can!
I have a two monitor system with an external Reference monitor as well, my main computer monitor started blinking yesterday and today it got really twitchy and nasty. I opened AJA Control Panel and selected Macintosh Desktop as Control instead of Black for the Reference monitor, so as to take the dying main monitor out of use and remove it, and then to use the Ref monitor as 2nd monitor until I can get another computer monitor tomorrow. But there was a blink and the Log-in window suddenly appeared on the Reference monitor that I had just set to "Mac Desktop" - but I was already logged in - and the system froze.
I powered down, disconnected the dying monitor.
I restarted from Diskwarrior disc and checked everything - all fine, rebooted and found that the Log-in window was no longer visible, my second monitor was still the 2nd monitor and nothing came up on the Reference monitor. I keyboard shutdown and swapped the monitor connector to the other socket on my graphics card, restarted - same thing again, 2nd monitor treatment, no Log-in window but logged in blind by keyboard but no Menubar or Dock is available so I can't change Display settings.
Shutdown and PRAM started, reboot and nothing has changed. Tried Open Firmware startup but this will not work, (is this an Intel thing? This is a MacPro DP 2.66 running 10.4.10) the computer simply goes to Desktop and Log-in is still not visible.
What else can I do? I don't want to reconnect the dodgy monitor in case it is this that caused the problem initially. And I worry that a new monitor may not work until I resolve this situation.

I seem to recall a really good discussion of just the solution you need in order to blindly reconfigure to Mirrored video output vice primary / secondary monitor output.

It involved using keyboard commands to navigate the menus required to make the change.

I'll go start a search on the forums and report back in a few....

...and here's the link.

I'll be moving your thread later today into the Peripherals & Handhelds forum where other such topics related to Monitors and such live.....not to worry, there will be a temporary pointer to the new location once it moves.

EDIT: added link to solution....

Thank you, it is absolutely brilliant - took me a couple of goes though, somehow the initial mouse position wouldn't give me Sys Prefs, but after that, fine. Thanks again, working again now.

Happy to hear a successful outcome.