What Countermeasures Can a Blockchain Implement to Recover from a Successful 51% Attack?

The primary countermeasure is a coordinated community response, typically involving a hard fork to a new, incompatible algorithm or a rollback of the malicious chain. A hard fork effectively "bricks" the attacker's specialized mining hardware, making the attack too costly to repeat.

This action requires consensus and is usually a last resort due to the complexity and disruption it causes.