Can an intermediary block be lost?
It is technically possible that all copies of the blockchain are lost...but extremely unlikely.  Also, I see no reason why such a massive data loss would be restricted to a single historical block, rather than the whole blockchain.

For a cool visualization of the number of full nodes on the network, look here:

https://bitnodes.21.co/nodes/network-map/

Keep in mind that there are probably a lot more copies of the entire blockchain than there are full nodes.  For example, I run 1 full node from my house, but also have 3 full copies of the blockchain backed up (so I don't have to download again).  It would take a cataclysmic event to destroy all the world's backups.

Also, having worked IT for a bank, I can assure you that your bank account doesn't have nearly this many redundant backups.