This happened to me yesterday. I was at work with the battery at about 50 percent. I was trying to access the Reddit app and it hung up on loading so I hit the home button and my phone went blank and wouldn't turn on for a few hours. I thought that maybe I was mistaken with the battery, but when I plugged it in... nothing.
I hit power plus down and I guess that worked to turn it on. Maybe. (It actually booted up on its own at lunch without me doing anything but when I tried to unlock it, it shut off again.) After turning on, I could receive calls and texts and connect to the internet through wifi, but it won't connect to the internet over 4g. (There's also no 4 displayed next to my bars anymore.)
Earlier this morning the phone also stopped recognizing the sim card and I had to reset for the phone to detect it. (This has happened once before a couple weeks ago.)
So... my phone is less than two months old. And from what it sounds like from the rest of this thread, I need to get it replaced?