My sister has an iPhone 3Gs. She's had it since November 2009, lately it hasn't been working. The screen is completely black and when she plugs it into the charger, it flashes every 20 or so seconds with the Apple symbol on the screen, but never turns on. I've tried resetting it using the sleep/wake and home button, but it just stays completely black with no reaction. It's been doing this for several months. Is there anything I could do to fix it? Thanks! 