Update: A Hearthstone hotfix is now in place, Ranked Play has been restored, and everything should be working as it's meant to. Naturally, Blizzard feels bad about the whole thing, and that's good news for you.
"As a token of our appreciation for your patience, we’d like to offer everyone three Kobolds and Catacombs card packs," community manager Aratil wrote. "These card packs will be distributed next week and can be redeemed via the Gifting function either on the Battle.net Desktop App or by logging into your Battle.net account on your mobile device."
One minor piece of collateral damage is that today's planned developer livestream to discuss the changes coming in the Year of the Raven has been cancelled. It'll now be rescheduled for a later date, meaning there's still time to submit your questions here.
If you've very recently found yourself unable to fire up a Ranked match in Hearthstone, don't panic. There's a problem of some sort in the March 2018 season roll, and so Blizzard has temporarily disabled Ranked Play.
We are aware of an issue with the March 2018 Ranked Play season roll. Please keep an eye on this thread for updates: https://t.co/rtGcO5IRVM pic.twitter.com/mgmQFFYIgbFebruary 28, 2018
"We are aware of an issue with Ranked Play progression for the March 2018 season roll. While we investigate, we have temporarily disabled Ranked Play mode in regions where the March season has begun, which currently includes Asia and Europe," community manager Keganbe wrote. "We will continue to update this thread as more information becomes available."
Blizzard hasn't cited a specific reason why the temporary lockout has been implemented, but it does seem to coincide with this Reddit post claiming that Chinese players were earning 5 stars per win rather than the usual one after the ladder reset. We've asked Blizzard to confirm whether this was the case and are awaiting an answer.
The March 2018 reset sees the biggest changes made to Hearthstone ladder since it began. Rank resets are being changed to become less punishing, but every advance in rank will require five stars rather than a level-dependent amount. The changes will make it tougher to climb through early ranks but should also result in greater differentiation of player skill between them.
Dropping only four ranks after each reset should also reduce the need to play "brutal aggro decks" in order to climb as fast as possible. We'll have to wait and see whether than proves true. For now, have fun in Casual!