Once you gain entry to the Labyrinth, it should always be open to you. What exactly is it doing or not doing that makes you think you can't get in?
It could be that you actually have a bug, in which case you should contact the Professors. In the meantime, you can still get in via a friend.
Thanks, I have finished questing for it yet again, so far it's working. Can't tell if it's the game or my machine. For some reason it seems like i'm setting back, not sure what to do about it. Maube it is the realm thing I heard about