When trying to connect an external screen (video projector) to my laptop (Dell Latitude E6530), I tried to switch to the external display using Settings Manager -> Display. I changed a setting, but I don't remember exactly which. Then, both screens went blank. Since then, after booting and logging in to gdm, the display is completely black. I am able to switch to the text console and applications appear to be running. The Ubuntu wiki Blank screen troubleshooting page states that:
If it occurs after entering your password on the login page, you have some different class of issue, such as an issue with 3D / DRM. Try disabling compiz (sudo chmod a-x /usr/bin/compiz), logging in as a different user, or turning off DRI.
I am not using compiz. I am able to log in as a guest user. No file in
/usr/share/X11/xorg.conf.d has a recent modification date. I am able to log in successfully in recovery mode, although I get once again a blank screen if I resume from suspension. Unlike other problems reporting blank screens after login (1, 2, 3, 4, 5), I have no mouse cursor. When in recovery mode, I can't change any settings through System Manager -> Display.
How do I repair my "normal boot" display configuration? Evidently nothing was changed in
/usr/share/X11/xorg.conf.d; where else should I search? What can I look for to investigate the cause in more detail?