Nodes can be online or offline. Online nodes are receiving, saving, and broadcasting all the latest blocks of transactions to and from other nodes, while offline nodes do not. When an offline node comes back online, it will need to catch up with the rest of the blockchain first by downloading all blocks that were added to the blockchain while the node was offline. This process is often referred to as synchronizing with the blockchain.
An entire blockchain can technically run on a single node, but since it would be stored on only a single device, it would be extremely vulnerable to things like power outages, hackers, or systemic crashes. The more Smart Nodes a blockchain has supporting it, the more resilient it will be against all types of risks. When the blockchain data is spread across multiple devices, it will be harder for a corrupt entity to alter or delete the ledger. Even if multiple nodes suddenly go offline and become inaccessible because of a global crisis, a single node can theoretically keep an entire blockchain operational. It only takes one node with the full blockchain history to come back online to make all the data accessible again.