Last night my phone was working perfectly fine... it had the latest updates of Graphene and around 80% battery.
This morning, I unlocked it, and the screen showed me a dialog that some app stopped working. I wasn't paying attention, I just clicked OK and I don't remember which app it was.
After that, I saw the "desktop" with app icons, and the phone was not reacting to any touch. I tried to block and unblock it with the power button, because I had this issue a few times before, and this always helped.
I blocked the phone and the screen went black, and then I could not unlock it. The phone stopped reacting to any input (screen, buttons).
I tried to plug it into a charger, and got no reaction.
I tried to plug it to my PC via USB, again no reaction (the phone didn't even show up on my PC).
I tried to hard-restart the phone by holding the power and the volume down buttons simultaneously for 7 seconds and got no reaction.
I tried several other methods of hard-restart that I found online (holding the power button for 30 seconds, etc.)
But regardless of my inputs, the phone seems to be restarting periodically by itself every 10 minutes or so.
Before it looked like a normal boot - first I saw the warning screen that the phone is loading a different operating system, then the google logo for several seconds, and then the screen went black again. This kept repeating.
And then on the warning screen, I managed to press the power button to "interrupt" the boot process. After that I saw the Google logo again, and after that the Graphene OS logo. I thought the OS has recovered, but after a few seconds of the Graphene logo, the screen goes black again.
So now the restarts keep repeating the same way as before, but now I get to also see the Graphene logo for a couple of seconds.