Why does my lumia 720 can't turn on?

[WPCentral Question]

hi guys. I upgraded my lumia to windows 10 a couple weeks ago. tbh it was very disappointing, so I did (some) hard reset, twice. a few days later, my phone started to act strangely. it won't turn on if I don't charge it, and once I charge my phone, it won't turn on. it did vibrate and nokia's logo appeared on screen, and low battery charging's logo appeared too. but after that, my phone restart and restarting again. I already did some soft reset, even the hard reset on button, and was trying to do some hopeless thing with exclamation mark. anybody know how to fix it?
note : sorry for bad english :")