After upgrading my Xubuntu 16.04 installation after a couple of weeks without updates, X11 was not able to start anymore. Only a black screen with a blinking cursor was shown. Ctrl-Alt-F1 to console worked but killed "X11". Easy solution: Remove oibaf drivers!

apt install ppa-purge
ppa-purge ppa:oibaf/graphics-drivers

Funny Keywords you may encounter during your voyage:

• lightdm fails, check service lightdm status, look into /var/log/syslog
• gpu-manager, either run from console via service gpu-manager
• console-kit-daemon: GLib-CRITICAL **: g_slice_set_config: assertion 'sys_page_size == 0' failed
• update-alternatives: error: no alternatives for x86_64-linux-gnu-gfxcore_conf
• /etc/modprobe.d is not a file (that is noise)
• gpu-manager Error: can't open /lib/modules/<your kernel version>/updates/dkms