Hi @ elolinon,
if you connect your charger to the phone, would it start after you press the power button or is only the notification LED red flashing?
In case only the LED is flashing, your battery could be discharged under a certain value where the charger won´t start the charging process.
That doesen´t always mean the battery is defective.
With a Multimeter you can measure the cell voltage.
Is the Voltage above 3V you can charge it with an professional external charger to above 3.8V.
Then the charger in the Fairphone kicks in again.
In case you do not have the equipment you can go to a repair cafe or to a RC-modell hobbyshop.
My battery(almost 2years) is still running because of this.
Try to end app processes which will deplete the battery to this state (eg. GPS navigation overnight on)