I have a Power Mac G4 with Mac OS 9.x connected to a LCD display, I changed the resolution to 1920x1080 but then the display went black, I waited but it didn't change back. So I restarted, the happy mac is visible then the display gets black again. When I start holding shift (extensions off) it boots with a different resolution which allows me to see the system, but I can't open the control panel as extensions are off. After restarting I get a black display again.
What can I do now?
It may or may not help, but try booting with Extensions Off and then moving the "Monitors Preferences" from System Folder: Preferences onto the Desktop. If when you reboot normally the display is fine, then you can delete that "Monitors Preferences" from the Desktop.
Normally when you change display resolutions the screen changes, waits for a few seconds displaying a message asking if the change is okay and if you click the button to say it's not okay, it switches back again. It will also switch back after the timer has run out without a response (i.e. because you can't see the window). It may be possible that the Mac crashed while in the middle of that and now thinks the new resolution is the correct one.
Boot with extensions off and then toss the display preferences (System/Preferences) file in the trash and reboot.
It's possible holding command option P R at boot (Pram reset) might also work.
@WhosIt.There:
That didn't help. 
I pressed esc after I changed the resolution and the screen went black but that didn't do anything. I then waited for about a minute and nothing happened. Then I pressed some keys on the keyboard and the system sounds played, so the Mac didn't crash.
@sfp1954:
You answered while I wrote the above answer, I will try your suggestion now, I tried pram reset already without any luck though.
@sfp1954:
Removing the Display Preferences didn't help, but removing the whole preferences folder did! Thanks. 
Weird thing is, after I changed the resolution again, the screen went black, I waited without doing anything but it didn't change back as it should. Now this will take some time to find a working resolution.
edit: I was wrong, removing "Display Preferences" helps. I removed the "Monitoreinstellungen" before, as it is a german Mac OS. Now with a clean preferences folder I saw that there actually is a file called "Display Preferences".
Look up the specs for the monitor are using. You want to find what the "native resolution" of the LCD is.
You shouldn't go any higher than that. Hopefully the LCD is Mac compatible. By that I mean that the native resolution is at an aspect ration of 4:3 (sometimes specified as 1.33 : 1). Mac used only 4:3 monitors (1024x768, 800x600, 640x480). 1280x960 might be available depending on what display card you have. If you have a PC LCD it may not support common Mac display settings.
I was going mention the file "Display Preferences" when I saw it in my System Folder, but since the Control Panel is called "Monitors" and there was a "Monitors Preferences", I thought "Display Preferences" must have been a left-over from OS upgrades or belong to a different program.
