My question is, after eveyrone is online, can the blockchain continue? Everyone just turns on their computer and continues working?
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.
Where is the blockchain stored? I always imagined it as an online spreadsheet that miners all over the world are "hacking"
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.
I don't think that it is a "physically" stored somewhere because everyone can see this since it is an open source program.
You are a senior member and you think that?
So that information you see with those block explorers, where does the data come from?