Hi there,
my FP3 has a very weired problem, I think related to the brightness Sensor.
After I Boot the phone, I See all the Boot animations and then it switches to where I have to enter my pin, and then the display just fades to completely black and I cannot do anything.
Is this a known problem where someone can help me? It started when I turned the screen brightness very low, so I think it is related somehow to wrong sensor readings.
Yes this was a problem, and there have been updates which resolved it and then invoked it again 
Go to a very, very dark place and sit for a while to get your eyes accustomed to the dark. Then you may be able to pull down the notification menu and see enough to brighten the screen
Go to Settings > Display and turn off Adaptive brightness
Check you have the latest OS update, assuming you are on the default A11
The latest public release is 8901.4.A.0019.1
Once you have that you can enable Adaptive brightness again.
The same thing happened to me, except on my FP4. I just kept rebooting my phone over and over again and eventually it corrected itself.
Hmm, so I was in a very dark room, but the display is just pitch black. Any suggestions? It needs to be absolutely pitch black? And I have had the latest update installed.
Not much? 
So have you held the power button off for at least ten seconds, or remove the battery for some similar time?
Then before you power on press the power button briefly to see the charge status.
It’s not a bad idea to have the phone plugged into power before you power on.
If you do get some response and the battery is not dead, if this doesn’t help you may be able to try a factory reset, this however will wipe all your data.
If that is true it may be a different problem 
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