Re: Graphical display is black after upgrade from Debian 7 to 8
I lost the thread already, sorry, but just in case you didn't know. Yuo probably have to debug this in 'single user mode' which you can get by grub, if the grub menu displays at all. (Try hitting escape, then chose 'advanced options' or maybe 'e' for edit to see the options).
Lookup the 'linux' line. Booting the 'single' entry, you hopefully get a root console. Now you can lookup the graphic card via 'lspci -v' (maybe send us the result) and you can install packages with apt-get. If you know how to use aptitude, install and use this.
If you install the meta-package xorg-driver-video then xorg will try to chose the best guess as driver. There are fallbacks like VESA framebuffer, so at least something should work.
Do 'less /etc/X11/xorg.conf' to see if there's a driver preconfigured. Maybe disable that by renaming the file so something like xorg.conf-suspendet and relaunch X. (Xorg doesn't need this file, it figures things out by itself, but you still can place 'override' defaults here)
by the way, if you need more extensive debugging you should temporarily disable the login manager (lightgdm or kdm or so) from the boot scripts. Today that's done via systemd 'systemctl' but i can't tell you exactly how, since i'm not using that init system. Without login manager, you can reboot and end hopefully up in textconsole login. Now you can login as root, and on another console (try Alt + arrows) as user. Just do 'startx' or 'xinit' from the user console, and immediately see the result.
It just comes to my minid, if you don't know how to disable services, it might be easier to temporarily deinstall the loginmanager completely.