What happens if one DC goes down in Cassandra multi DC setup?
All writes are always happens to all DCs (see the diagram in DataStax Architecture guide).

But if the one DC is down, the nodes in the other DC will detect this, and will start to collect hints during the configured hints time window (3 hours by default, and cross-DC hints aren't disabled) - if servers in first DC comes back during this window, hints will be replayed