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Could you poen your Macintosh HD and go to /Users/<yourusername>/Library/Preferences/ByHost and remove the following file then reboot:com.apple.preference.displays.<stringofLetters&Numbers>.plistLet us know what happens.
In that case could you reboot your mac and as soon as you let go of the power button pres and hold the shift key down till your machine opens in Safe Boot, This may take a little while so be paitent.
Once the machine is open in Safe Boot go to the Apple preferences menu and set the resolution to the one you wish to use as default.
Reboot the machine like normal and let me know what happens.
Sorry I missed your last post.
Did it come back ok after the reboot?
Reboot, Restart, Soft Boot are all the same thing.