There are lots of reports that Ubuntu 17.10 hangs with a black screen if reboot is issued. Some users report they need to press Alt+F7 to go further, some users report it only takes a looong time to go through the reboot sequence. I could confirm both issues, luckily there is help! Edit /etc/default/grub and remove quiet and splash from GRUB_CMDLINE_LINUX_DEFAULT. Issue a sudo update-grub and reboot.