I'm running Debian Sid on a laptop. I usually connect a external monitor
to the HDMI output of the integrated graphic card (an Intel, if relevant).
When I'm at home, it works fine, but it is not working at work since a week.
When I start with the monitor connected, I can see the manufacturer
splash screen, GRUB and the first messages at the start up on my
external monitor, but after a few seconds, when the screen goes to black
and the terminal changes to "graphical" mode, I lost my external monitor
connection and only can use the laptop screen.
At home, in the same point where I lost the external monitor at work, it
began to see the startup messages mirrored at the two screens.
I have look at /var/log/messages looking for some HDMI-related error
messages, and the most likely I can found is this: