My phone has suddenly turned off when I was not using

[AlexH]

I haven't used my phone for about 30 minutes and I just tried to use it and it was turned off.

  I tried to turn it on with the power button but it didn't work.

  The battery level was at 38% before it turned off on its own, but just in case, I connected the phone to the charger and a few seconds later the phone powered on.

  I don't know why this happens. Is this a serious problem?

[Mattias_ASUS]

Hello @AlexH 
I recommend trying holding the volume down button and the power button at the same time for about 20 seconds.
This will hard reset your phone.

[AlexH]

I think I did that when I plugged my phone so maybe that's how the phone powered on.