Yesterday, August 1, we made a hotfix to the game that resulted in an unexpected bug for a small number of players. Those affected lost some or all of the quests from their quest log when they moved into or out of an instance. We quickly discovered the issue and put a stop to it.
The timeframe during which it was possible for players to be affected by this issue was 12:34 p.m. PDT to 2:48 p.m. PDT for Asian realms, and 12:34 p.m. PDT to 4:18 p.m. PDT for Americas and European realms.
If you were in-game during that timeframe, and you lost quests from your quest log due to this bug, you should be able to go to the relevant quest givers and re-accept the quests.
It is possible that you lost quests that can't be re-accepted. These might include multi-week quests, or quests that are no longer available. While we expect that re-accepting most multi-week quests will also restore your progress, it may not. In those cases, we suggest you contact Blizzard Customer Support so that we can look into correcting your character’s quest log.
We apologize for the trouble, and we appreciate your patience as we work to bring affected players back to the adventures they expect to be on.