How to Configure A New Monitor As the Only Monitor

I'm using Ubuntu 20.04.3 LTS and I'm not familiar with X. When I installed the OS, I was temporarily using an old monitor, with an HDMI to VGA adapter. Then, I bought a new monitor and found that Ubuntu gave me a black screen on the new monitor. Later, I found that before I login, it can only display on the old monitor and after logging in, it can display normally on my new monitor. What I have to do now is to plug in the old monitor, log in, and unplug the old monitor, switch to the new one (with a single HDMI port on the graphics card).

What I suspect is that Ubuntu "remembers" the old monitor and treats it as the primary monitor, so it displays login screen only on the old one.

I'm wondering is there any way to fix this issue?

Sometimes the easiest fix is to reset xrandr. Here's how:

If you are booting with the new display connected:

0. When your system finishes booting and goes to a black screen, press Ctrl+Alt+F1.
   ^{Note: You should be able to use any of the F# keys for this, but F1 is a fine place to start.}
1. Log into the system via the console
2. Move on to the "Reset xrandr" section of this answer

If you are booting with the previous display connected:

0. Boot normally
1. Sign in normally
2. Open Terminal

Reset xrandr:

0. Reset xrandr:

   xrandr -s 0
   

1. There is no further step ... hopefully.

What this does:

From the documentation regarding the -s option:

-s, --size size-index or --size widthxheight
    This sets the screen size, either matching by size or using the index into the list
    of available sizes.

So -s 0 will reset the connected display to use the first available size identified via the HDMI handshake. If you would like to see a full list of display resolutions available, you can do so like this:

xrandr -d :0

Which will give you something like this:

Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 16384 x 16384
eDP-1 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 309mm x 174mm
   1920x1080     60.05*+  60.01    59.97    59.96    59.93  
   1680x1050     59.95    59.88  
   1600x1024     60.17  
   1400x1050     59.98  
   1600x900      59.99    59.94    59.95    59.82  
   1280x1024     60.02  
   ...
DP-1 disconnected (normal left inverted right x axis y axis)
HDMI-1 disconnected (normal left inverted right x axis y axis)
DP-2 disconnected (normal left inverted right x axis y axis)

If your graphics adapter has just a single HDMI output, then you will likely see information just for HDMI-1.