Yes, unfortunately every registered usernames are lost.
This means that all previously registered usernames are now available to be registered again.
However contacts are not internally identified by usernames but using public keys, the registered name is used for human display purposes only.
This means that contacts don't need to be re-added again.
When the local name cache is cleared, contacts are presented using their public key fingerprint, or their new username when they register one.
Sorry for the inconvenience, as mentioned this was caused by data loss on the only running blockchain node.
We work to make sure this won't happen again. When many blockchain nodes will be ran by the community it won't even be possible for us or anyone to reset the name directory.
Ring project director,