It'll be there waiting for you when you next log in.
If you logged into Hearthstone today only to discover an unopened Frozen Throne card pack waiting for you, then you were affected by a bug that hit a small number of players at the launch of Knights of the Frozen Throne.
Blizzard recently introduced a change to the card pack algorithm, so that you’re now guaranteed to receive a Legendary when opening the first ten packs of any given card set. Some players apparently went eleven packs before receiving their guaranteed top-tier card the other week though, hence the compensation today.
If you don’t see a freshly minted pack when you next log-in, you weren’t affected by the bug. There’s no in-game explanation for this sudden burst of generosity from Blizzard, but an email was sent to the registered address of affected accounts.