From their website and most likely related.

Scheduled system outage: April 18-19
Our scheduled system outage is complete, but booking of new tickets on HawaiianAirlines.com is not available as we bring our systems back online. You may book new tickets through the Hawaiian Airlines app or your preferred online travel agency.

You may now use the mobile app to check in for your flight or see an agent at the airport. We are currently experiencing intermittent errors with check-in on our website.

Our contact center is experiencing high call volume, and you may experience longer than normal hold times. Thank you for your patience and please click here for more information on our technology upgrade.