@tphysm: Thanks a lot for changing the title of this thread - hit on for me to get my phone back into functioning!!!
I had one of those crashes this morning, and another one last week. Using a 4 g network (virgin mobile FR), and our home wifi says 5ghz (Bouygues in France). And only one SIM card, on a FP2. Not really changed anything on the delivered system.
screen went all black and no reaction on touching screen or buttons, while I had phoned a landline number and wanted to hang up while the voicemail set in. I don't recall having to re-set it, I think functionality simply came back a couple of moments later.
Not sure this is really related to the issue discussed here, but was a first go-black for me...
No action on my phone for a while (2 hours or so), then wanted to send an SMS and discovered my phone had gone all black with no reaction whatsoever, not even when putting it back on charge, no LED signal that it was charging again/ green or was out of charge/ red, simply nothing. (It had fully charged this morning).
Thanks to the hint to press the power-button longer than 10 seconds, I got it back into use and finally could send my SMS.
I did watch a couple of videos on websites (via wifi!) on my phone this morning, so possibly just before this happened, but cannot really say as discovered only later the unresponsive screen/phone.
No heating up for me (at least didn't realise).
I am wondering whether it has got anything to do with the function to limit background data usage (achtergrondgegevens beperkt, in Dutch), as this was a first time this happened to me. I only recently discovered this function, and after this mornings problem it was deactivated. (Not sure this is normal after a reboot though.)