Fairphone 4 suddenly shut down and won't restart

If you’re sure that its not just the screen that’s dead, it honestly sounds like a core module issue. I wonder wether that’s a problem similar to the FP3 sudden death phenomenon.

If you still have warranty the best way to go is probably to contactsupport and get a replacement. If on the other hand you’re still looking to recover data, then some of the troubleshooting steps suggested for the referenced FP3 problem (such as performing a reflow to fix damaged soldering joints on the core module) may help, but beware that that’s rather a last-ditch effort than a first step and may also damage other parts if unsuccessful: