This is a problem I've experienced twice now.
The problem occured first time, a day after I had installed my new R9 390 GPU (which is about a month ago now).
I was playing a game and watching a livestream, and both monitors would all of a sudden turn black (no signal), and the PC kept running.
This happened while I was still on Windows 7, so I tried to make sure all drivers were up to date, swapped out a power cable to the GPU (thinking that it was getting incorrect power), and then nothing else happened since then..
Until today. I was not near my computer, only having a livestream running on my second monitor, when I see that both monitors turn black with no signal, exactly the same way as first time. This time I'm sitting on a Windows 10, clean installed not long ago, with all drivers up to date.
Both times I've needed to shut down the PC completely (Holding down the power button), before the computer would show any signal on the screen again.
I'm not exactly sure if the GPU is the problem here, if it's not getting sufficient amount of power etc. I cant seem to find a proper answer for this, and I've been googling around trying to find answers to other peoples problems that looked like this, but I'm really not sure what the problem can be, as it has only happened twice so far.
Worth mentioning as well, over the past few days, I've experienced a few times, where my PC would freeze (totally lock up) and I'd have to restart my PC to get it back working. I'm not sure if this some indication on, that my GPU is faulty or not.