Hi gap736uk
not sure if this helps but what you described sounds to me similar to a case which I had lately on my partner´s mobile - a Nokia 5.4
From one day to the next the almost fresh device (still in guarantee time; below 1/2 year) went black and couldn´t be started anymore. Also while plugging the charger cable in didn´t seem to work (no reaction like charge light). Last but not least the device didn´t start anymore and also connecting it to a pc didn´t show up any sort of connection as if the device was really dead.
So we sent it back on guarantee. A few days later the retailed sent us back the device - working again. Along with it they reported the device had a ‘depth discharge’.
I can only assume that they kept it charging even while the charge light and display stayed black until the encessary loading level of the battery was high enough again (since Nokia’s battery can´t be exchanged easily like the FP3)
So maybe giving it a long time to load may change the issue on your side.
Alternatively -if possible- it could be helpful to check if this issue can be fixed if you exchange the battery with another one (take a look about the #fairphoneangels
. Maybe there´s one near you where you can easily exchange and test if it´s battery-related …)
Hope this helps …
Best
DonFnord