There are five or six reasons you can't get into the Labyrinth. Since I don't know which one applies to you, I can only offer general advice.
Wizard101 is strict about following the storyline. There should be at least one quest in your list that has something to do with Dragonspyre. I suggest that you check the list and if needed the dialog (the question mark for a particular quest) to find out what is needed.
If you have a quest that asks you to "Explore the Necropolis", you need to run there through the Plaza of Conquests (this only has to be done once) to unlock the Necropolis portal in the Basilica.