Why isnt my phone turning on?

I let my Fairphone 2 battery die out and when I went to charge it again, it wont turn back on. I have let it charge for a whole day and it still isnt turning on. I have tried multiple chargers as well that are the spec Fairphone ask for. Sometimes it will go to the fairphone load screen and start showing the battery charge screen and says that it has 0% battery. After showing that the screen just goes blank.

when the charger is in the phone the red led light on the front is on, but that is even on when the battery is out.

is this a problem with the battery or a problem with the phone? My phone is only 1 month old.