Ubuntu only starts with an external connected screen? Is the HDMI cable unplugged?

I just installed Ubuntu while an HDMI cable was plugged into the laptop which led to a screen. It all worked out pretty well, except that I didn't see anything on the screen, except the mouse. So he recognized it somehow, but no programs / windows or the background were visible. The problem now is that the laptop goes out as soon as I unplug the HDMI cable. A terminal is opened briefly and something is output and then it goes out.

If I boot up the laptop without the HDMI cable plugged in, Ubuntu is not booted at all, but "recovering journal" is output. Then a few thousand / a few thousand files are cleaned (or the cleaned ones are displayed, I think) and a few thousand blocks / a few thousand blocks. After a few seconds it shuts down again, so I don't even get to login. What could that be? How could I fix it?

Try to assign the screen in the system administration.

Already tried without success.