Black screen laptop only one user?

On my laptop, there are two users and one can log in properly. In the other case, after entering the password, the screen is black and you have to close the laptop again, so that something goes again. The mouse can still move, but the screen is just black and you can't do anything.

Unfortunately, the user, where the screen is black is much more important and I cry when I think that now maybe everything is lost there.

Please give me some tips on what I can do! With one user everything works normally, only with the other not. I'm desperate!

As admin access the files of the other user. Or: Insert Linux ISO USB and copy files.