Moto X2 shows green led light and wont turn on

I tried switching on my Moto X after 3 months. I plugged on a charger and it showed a green led burning for 5 seconds and the screen flashed with empty battery symbol. I let it charge for a couple of hours and tried it again only to see the same process repeating.

I understand that this is a dead battery situation and the green led indicates that. How do I come out of this ?