Have you changed you monitor and you can’t access your desktop after login because there is only a blank screen?

It is caused by the Xfce display manager which is trying to set a different resolution and/or refresh rate. The monitor does not like it and it shuts down. Simplest solution is to edit/delete file

~/.config/xfce4/xfconf/xfce-perchannel-xml/displays.xml

It will reset the settings and voila – it works again!