Why is my nokia lumia not working?

please help!!! i have a lumia 635. actually, it showed warnings to charge the phone. i was busy and i left it like that. after a while, i've noticed that my lumia was strucked and it showed setting and power symbol. i tried to switch on the mobile and i've charged it too. even after all these trails it's not responding. i want to know the main reason for this and please help me to fix this issue