This may not be very helpful but I think it’s called the ‘black screen of death’, which can be very scary because it just happened to my iphone xs max last night. Most phones have a hard reset thingy (otherwise called a forced restart) that you can do by following steps and pressing buttons in a certain order, and it can fix the software problem. hard resets are different to factory resets because they keep all of the data and content on your phone, whereas a factory reset gets rid of everything and turns it back to how it originally was when it was sold. I’m not entirely sure if this is the same for a fairphone 3.
Maybe try this link:
https://howto-android.com/how-to-fix-fairphone-3-black-screen/
From what I know, it’s either a software or hardware issue. You can tell which one it is based on whether there is any physical damage or leakage, or if your phone is still showing signs of life (making sounds and vibrations when you charge it, etc). If I were you, I’d try putting the battery back in and charging the phone for at least 15 minutes or so and then try follow the steps for a hard reset. But if there is a physical hardware issue, you should probably get it to a technician and check if your warranty is still valid.
I don’t know much about androids so you probably know more than I do. Hope this helps and goodluck!