I have a Dell Latitude 5400. I'm running a secondary monitor using a HDMI cable that connects to the back of a monitor via an RGB connection.
Until this morning. It just went black and the monitor says, "No connection detected."
I shut my laptop down and rebooted. No change.
I disconnected the cables and reconnected them. No change.
I'm using this cable:
...and my monitor is an ASUS monitor and I get you some more specifics if you need them.
The bottom line is that it worked and then, suddenly, it did not.
I will add that this is a work computer and I don't have administrative rights. Not sure if that makes a difference, but there's that.
One more thing: Here's a screen shot of my "Display" settings: