Comment 5 for bug 1419507

This still happens on 15.04. I have a ThinkPad E540 with Optimus graphics. Plugging in a HDMI display causes display to go blank.

I found this blog posting which explains that the issue is related to gpu-manager not working correctly:

http://vxlabs.com/2015/02/05/solving-the-ubuntu-14-04-nvidia-346-nvidia-prime-black-screen-issue/

This bug is a serious for people using laptops with NVidia Optimus as it's not possible to connect a HDMI monitor to the device. The solution in post above is way too complicated to be a good workaround.