In a situation like this, there should be a "day zero," which means that Bitcoin should stop being utilized regardless of whether half of the world is still without electricity. When it is restored, it should make up for that moment of failure.  By doing this, you can prevent 51% attacks or low-difficulty block mining, which could be disastrous, similar to inadvertently offering a product at a discount in a store.  The Bitcoin network would supposedly "mark" a block as a secure reference point on a regular basis.  This marked block would serve as a worldwide agreement regarding the chain's current state.  in the case of a catastrophic incident, such a 51% attack or a widespread outage.