I've seen a number of discussions on what appears to be a very similar problem, but no real solutions. I thought I'd try again with maybe a bit more detail.
I have a Latitude 7470 running Windows 10. Display adapter is the built-in Intel HD Graphics 520, driver version 220.127.116.1127 (3/9/2017). I'm not using a docking station.
I have three monitors connected:
Monitor 1 is the laptop's built-in monitor
Monitor 2 is a a Dell E2416H (Display port)
Monitor 3 is a ViewSonic VG2228wm (HDMI to DVI cable).
Desktop is extended on all three monitors. Monitor 1 is designated as the main display.
At times (unpredictably), monitor 2 (the Dell monitor) doesn't wake up with the computer and the ViewSonic monitor. Power LED is blinking slowly. Apps that were previously running on that monitor are moved to one of the remaining two. If I power the monitor off/on, the computer signals that a new device was discovered and everything goes back to normal without further intervention.
How on Earth do you even start a new thread in these bloody forums?
Where's the big giant button that every online community that has ever existed has to start a new thread? I'm forced to post here with something completely unrelated to the OP because for the life of me I cannot figure out how to start a new post.
Assuming that you have a laptop video issue...
1. Go here for the main Forum page
3. From the three boards at the top, click Video
Try installing the latest BIOS and DisplayPort MST hub firmware update for your system; the latter pertains to a built-in device, not an external peripheral as is usually the case when discussing MST hubs. Both updates are available on support.dell.com.