First, all the events on the blockchain aren't lost if you delete a single node's directory, because other nodes have a full copy of the blockchain. But it is vital to back up a node's private keys which other nodes do not have.
The only exception is if some data was published off-chain in a stream and is only held by a single node, since no other node subscribed to that stream., then that data will be lost if the publishing node's directory is lost.
There is detailed information about backing up and restoring private keys and nodes here:
https://www.multichain.com/developers/backing-up-restoring-nodes/