Besides some bitcoin node(s) running on solar power, what happens if the network goes offline? What could get corrupted?

In the following Reddit post https://redd.it/2fz43q which poses the question of what would happen to the bitcoin network if it suffered a global power grid outage, the most upvoted answer mentions of a node running on solar. However, during the offline time, would it be possible for a portion of miners running on solar power to collude and carry out a 51% attack if the remainder of miners that should defend don't have the grid power needed to operate their nodes? I'm interested to hear peoples feedback and see which answer gets the most upvotes and whether it's accurate.

This concerns me because if there is some global power outage, irrespective of other damage it would cause elsewhere, I am curious if the immutability of historical blockchain transactions could be compromised to a degree that could alter its history to an unrecoverable degree once the network is fully back on power.