Problem was that my phone had been left in the rain. After days in rice, i found the battery wouldn't hold charge, and the power button didn't work (the phone would turn on when charging, so I could confirm that everything else was working, but could not induce sleep mode or turn it off using power button).
Repair involved researching on Ifixit.com to determine the problem, then ordering parts (repair kit, new batt, new power sensor cable), then following through with the online guides.
It worked! The power button functions again, the battery is better but I'm not really impressed with its quality. I'll get about a day from one charge.
Advice: follow the guides CAREFULLY! The high res photos were critical, tapping on them opens them in a new browser tab with better resolution, so you can zoom way in. Many steps have multiple photos, with one of them prompted above the text. Pan right to find tumbnails of other pics for that step. In short, patience and organization are essential. Booze is a no-no until its all over.