macbook external display issues

I've used my late 2009 macbook with my 23 inch wide screen monitor for a few weeks now and recently adjusted settings and came across an issue and im drawing a blank when it comes to correcting it.

I set it to mirror display I believe and since then my screen goes blank when connected to my external monitor and my external just shows random crazy colors and patterns. I have noticed the colors and patterns on the screen respond to my mouse movements I just cannot change the setting back to the original setting because that specific check box only appears when you connect to an external display and cannot be adjusted until it is...

Any thought are much appreciated.

When you set an external display to "mirror" your internal display on the MacBook, it will default to the resolution and settings of the internal display.

It sounds like your external display has become confused and out of sync. Perhaps you adjusted some setting such as the resolution or refresh that is not correct. (Also check the cables to be sure they're attached firmly.)

Remove the monitor from the MacBook and then set the controls to factory default. Turn it off for a bit and on again. You didn't mention how you have it hooked up but for best results you should use MDP to DVI.

Your correct I didnt say how its connected but it is MDP to DVI. I've tried connecting it to my l.e.d. monitor thinking it may be my lcd monitor or cable, and all was well until I selected the mirror display check box, then it did exactly the same thing..

Im not sure how to go about putting the monitor setting on my macbook back to factory default. But im assuming it has a memory of displays its been connected to prior and it switches back to the settings you enable at that time, since it wont respond the same way to a newly connected monitor.

Is there a way i can set all monitor settings back to factory without a fresh OSX install?

Is there a way i can set all monitor settings back to factory without a fresh OSX install?

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Try resetting the SMC on your machine. LINK If that doesn't seem to work, also reset the PRAM - NVRAM. LINK

Also, do you have another set of cables that you can try? Just in case the adapter or cable is acting up.