I have a dual-boot installation and after choosing Ubuntu in the GRUB menu both the built-in and external monitor displays go black if the HDMI cable is connected to the laptop. If I wait about 30 seconds and press Ctrl + Alt + Delete I can boot normally. I can't access ttys when the displays go black. If I boot up without the HDMI cable connected everything run as normal and if I connect the cable after the boot I have no problem with the second screen.
I am using Ubuntu 18.04 on a lenovo ideapad 700 with gnome 3.28.2. My kernel version is 4.15.0-42-generic. I am using nouveau drivers.