Every full node has a full copy of the block chain.
Once internet is back on nodes will begin again communicating and miners will start adding blocks to the chain.
The only drawback is that somebody might take advantage of the low number of full nodes and try a sybil attack.
Could it be easily reset once everyone got back online?
Miners are not "hacking" they are just adding blocks with transactions to the chain.
There is no reset. It will continue from the last transmitted block.
You are a senior member and you think that?
So that information you see with those block explorers, where does the data come from?