It's impossible to abandon a class story, you may have confused something. But in any case, tomorrow's patch (4.0.2) will restore any missing class quests, so check if you have the class quest again after the patch.
If you don't and you are a subscriber, you'll have to put in an in-game ticket. Customer Service can manually fix class quests (but only class quests) but make sure to include as many details as possible, including your character name, your server name and the name of the quest your abandoned.
