Non-rooted LG G3 D855 suddenly stuck on boot / can't get into recovery
Last night at a party I go to take a picture by holding down the Volume Down button whilst locked.The camera app didn't pop up and subsequently my phone crashed. I thought nothing of it, but then it wouldn't turn off or recognise anything. Even when I popped the battery out and in. It's stuck on boot. So I let it run out of charge, go home, charge it and the little battery icon animation that goes round once you've turned it on gets stuck, I can't tell what the phone's percentage is - it just stays frozen. Stuck on this now instead of boot.
After about 10 minutes of it being stuck suddenly I got the error message:
 Fastboot mode started
?!?! What gives? I only rooted it once but then flashed it to stock immediately when the phone first came out and I've left it as is since.
Any help is really appreciated. I need my phone for work tomorrow.