Why is my phone not turning on?

Why is my phone not turning on?

The phone is not turning on. Not even the screen or the led lights. I don't have the original charger so I'm​ using a 4-6V charger with 800mA output. When i check the voltage of the battery, it shows slight increase after a long time. So i know the battery is charging. Yet the phone is not turning on. What might be the reason for this?