DIAGNOSIS This is almost certainly the Ambient IQ light sensor doing its job a bit too aggressively. It's dimming the screen down to what it thinks is an appropriate level for your room's lighting, sometimes all the way to black.
FIX First, just give the top-right edge of the screen frame a firm tap with your finger. That's the sensor location, and a tap can sometimes jolt it back to sense correctly. If that doesn't do it, go into the settings on the device. Swipe down from the top, tap the gear icon, then go to Display. Look for 'Ambient EQ' and turn it off. Your screen brightness should immediately return to normal.
IF THAT DIDN'T WORK If the screen is still black but functional, do a hard reboot. Press and hold the volume down button on the back for about 10 seconds until you feel it vibrate and it restarts. That clears any temporary glitch holding the display back.
HEADS UP With Ambient EQ off, you'll have to manually adjust brightness in the Display settings if the room light changes. It's a trade-off for a reliably visible screen.