Try this. Go into your quest log, and click a quest for the world you are currently in. Finish it. Go to the guy who gave you it and get your reward. If he has another quest and you get it automatically and you can't decline it that means its your main quest. if none of your quest is your main quest then you lost your quest. to solve this. Go into the world you are in. and keep doing quest finder in till you find your main quest. hope this helped!
What area of Wizard101 are you currently in? By which I mean, what world? Wizard city? Dragonspyre? You can see all of the "main" quests for your current world by searching a quest tree. For example,
I don't think I "lost" my main quest, but I think I don't know which one it is..can ya help me out?
-Grandmaster Sorcer Brandon StoryRunner
The one you can't deny is your main quest.
However, I think there should be a quest organization system like on Pirate101. There should be a Power Pip icon for the main quest, a normal pip for a side quest, and a card for spell quests.