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.