So my phone simply is off without cause

This is the background of my issue:

I have an iPhone 3gS. I had a picture on my phone I was going to upload to facebook, but needed to e-mail the picture beforehand to then upload it to facebook. As most iPhone users know, e-mailing a picture from the iPhone takes a good 20 seconds. So, as a user who has the setting that auto-idles your iPhone after x minutes toggled off, I just left my phone on the other side of the room, which was at ~35% battery when I did so. I came back from the restroom about a minute later and the phone was simply off.

I've tried holding buttons down, tried plugging it into both computer and wall outlet for 10 minutes each without anything new.

I've never had any issues with my iPhone in the past.

Any tips are welcome and appreciated.

 

have done that in the past 20 minutes without result.