1-check battery using multimeter (voltage should be around 3.8~3.7 on DCV range)
2-if battery is not as mentioned above connect it to a power supply rated at 3.8v (+to+ & -to-) for about 1 min then measure its voltage again.
3-if battery voltage increased then it is ok and accepting charging but it was flat dead.
4-if not then battery is damaged and need replacement.
you can also try another procedure as follows:
1-plug your charger in wall outlet, don't connect its usb to your phone yet.
2-press and hold power button
3-while like that....plug in charger usb to phone socket
4-release power button on sight of starting screen