HDMI connection to external monitor causes laptop screen blackout in 14.04, nvidia graphics
Does HDMI1 and LVDS1 both show when you use xrandr in terminal?
If so you may be able to temporarily workaround it by using
xrandr --output LVDS1 --auto
If there's no errors the display should turn back on.
Also if you want xrandr with a gui you can use arandr:
sudo apt-get install arandr
I had the same "problem" (regarding the laptop lid), the solution is simple: you have to raise the screen brightness, I don't know why but when I plug the HDMI the brightness of the laptop lid is set to 0 and it seems that the screen is turned off.