My FP2 is one of the original pre-orders, I was never able to turn on the screen and actually use this phone. First I assumed it could be a display problem, so I received a new display but still no change.
Well since you were able to update the OS via fastboot I’d rule out a broken battery or a generally broken main module, so there are two options left: you got another broken display or your main module’s connector is broken.
If there is a group of #fairphoneangels near you they can probably help you determine what is broken by switching modules.
Otherwise you’ll probably have to send the phone in for repair. Here is how you best contact support to get a fast reply.
Where in Belgium are you? We could write something like “can anyone in XY help?” into the topic title. There may not be an angel group yet, but there are surely some helpful Belgians active on the forum.
Without testing with different modules I don’t think it’s possible to determine which module is broken (unless of course it is visibly obvious).