iPhone reboot loop, can't connect to computer

I recently acquired an iPhone 4S (A1387) from one of my teachers, and it was in pretty rough shape. The screen is severely cracked to the point where the main logic board is visible. I tried plugging it in to my computer but the screen wouldn't turn on. When I plugged it into a 10W iPad charging brick, it started booting, and then turned off again. It did this a few times until it finally was able to get onto the lock screen. But then it just turned off again. I found that if I held down the power and sleep buttons at the same time, it wouldn't turn off while it was still plugged in. I verified that the cable is OK with an iPhone 3G. There is a password on the phone but my teacher doesn't know it. I think I might need to restore it without the computer.