I have a Macbook Pro 15inch Retina mid-2014. I always use an external display when not on the move.
When I plug in an external display, at home or at work, both the built-in display and the external display go blank. They are both backlit and the monitor does not go to power saving mode. I can hear notifications arriving, so it's not a freeze or panic.
This happens when using either Thunderbolt port with TB to DisplayPort cable. My displays are a 1440p HP Omen and a 4k AOC. Happens on both.
Unplugging display leaves built-in display blank, so a force reboot is required.
I am going to check if HDMI works better, but would rather the MacBook work as expected.
Edit Displays work as expected after a fresh reboot (or a force-reboot).