this sounds like a battery issue or dock connector issue. Clean the USB port of any dirt and dust then put the phone on charge via a wall charger not the pc. If it doesn't charge at all after an hour then you'll need to open the phone and swap the battery... pretty straightforward soldering job. The tricky bit is the removing of the case etc to get to your battery. I would think that it will be battery as it is approx 3 years old now? and they won't run forever...