If this happens all of a sudden, I'd wonder if someone is trying to do a 51% attack with the remaining hash rate.

In this scenario, a 51% attack becomes very likely. What if there's no global blackout, but instead, your country got disconnected from the rest of the internet? The miners in your country will continue, and maybe make up 5/10/20% of global hash rate, but the rest of the world will mine much faster. So the moment the internet is restored, you won't have the longest chain, and your latest blocks will be orphaned and all transactions in their will be reorganised.