iPhone: Black screen

I have am iPhone 6s. Yesterday all of a sudden the screen went black and the phone didn’t react anymore. I tried to connect it to iTunes via lightning cable, which worked. The phone synced and also made a backup. So it was still turned on.
I then force-restarted it and it worked totally fine after that.
Any idea what was wrong with it? Will that happen again?

My work phone is an iPhone 6s also. I have had that same situation like 3 times in 2 years. Not sure why but i never gave it a lot of thought, just hard reboot and keep going.

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