Hi!
2 days ago my Fairphone 3+ also suddenly stopped working. I reflowed the motherboard as explained by @HolosericaCaligo and it works perfectly now! 
Thanks!
I put the motherboard 15min the oven at 200°C (preheated) in a glass oven tray (pins up!). I let it cool down (~30min) and rebuilt my phone.
I found it pretty easy to do. The last 2 screws (the ones holding the motherboard) can’t be unscrewed with the Fairphone screwdriver, you need a very small torx.
Good luck to those who will try as well!
Side note: since my phone was open, I cleaned it and especially the bottom speaker. It incredibly increased the volume and quality of the sound. Sounds logical, but I wouldn’t have spontaneously thought of cleaning it.