So can a blockchain can be shutdown by shutting down power supply across the globe? What would happen in such a case?
If the world's power supply was suddenly cut off, then everything electronic would stop working. The blockchain would be 'frozen', each node would retain its local copy, but be unable to communicate with other nodes, and miners would be unable to mine and add new blocks.
I think the transactions that were not added to the ledger would definitely be lost and they would have to be reinitiated.
This depends on what happens to the mempool of each node. If any nodes remembered any unconfirmed transactions from before the power loss, they could simply broadcast those transactions to the network once things went back online.
I think power shutdown would create inconsistency between different instances of the ledger at different nodes and even if the power is back we wouldn't be able to pick a safe state of the ledger.
Each individual node, once it came back online, would reconnect to some peers in the network, and listen for information about new blocks, just like normal. The decision about which chain to follow would also be made as per normal: which chain has the most work? Follow that one. If there are multiple choices for a most-work chain, then the decision about which one to follow will be made when the node hears about a new block, which extends one of those chains (the other chain will become 'stale', and will not be a part of the blockchain history).
What are the possible ways to shutdown a blockchain without having to go to each and every node and taking it down?
This is a strength of the bitcoin network: it is a decentralized network of computers, without any real hierarchy or centralized control. You would have to attack every node, otherwise the un-attacked nodes will be able to maintain the network. Given the highly distributed and anonymous nature of the network, this seems to be an extremely difficult task.