I have been dealing with the black screen issue on the S5 for a couple of weeks now. It started happening after the Marshmallow update and, interestingly, a few weeks after the warranty expired. If you Google "S5 black screen", you will get a lot of posts telling you to do a factory reset and a few Youtube videos instructing you to remove the battery and reinstall it.
It started as a black screen once a week, so I was doing the battery removal. Then it was doing it every 10 minutes, so I did the factory reset. After the reset, it was doing it once/twice a day, so I followed Verizon Support's advice and ran through the Samsung Software Repair utility. Less than an hour after the repair, it went to black screen again. I used the 3 button reboot method (Power+Volume Up+Home Screen) to restart the phone. Be careful using the 3 button method, you might inadvertently do another factory reset

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The phone was good for two days, then started doing it again. After another chat with Verizon, they are sending me a new S5 that I should have today.
After they shipped the new phone, I discovered something. When the screen goes black, rapidly press (for those who remember, think Asteroids from the 80's) the power button 8 to 12 times, the screen may come back! It's a lot less hassle than pulling the battery...
Hope it helps.