The easiest way to get the monitor back on is just to restart the entire machine and 99% of the time it comes back on. I have the laptop running on the integrated Intel graphics but the same thing appears to happen on the Nvidia graphics as well. The monitor appears to wake up (it turns on) but it flicks between the sources and reports 'no signal'.
Sometimes both of them do but that only seems to happen around 1/10 times. However, if I put the computer to sleep and then try and wake it up again, only one of the two external monitors comes on. Everything works fine on first boot: I turn on the laptop, type in my password, shut the lid and just use the two external monitors. My current setup is my Dell XPS 9560 (1080p) with 2x 1440p 75hz monitors.
