It simply means that you have lost it.
There might be two reasons:
While being ranked as beginner, a knowledgeable member who has observed your behavior and knowledge level found you as fluffy. So when you are marked as fluffy, your beginner or any other previous reputation is gone, and the person who owns the account can see a blank space instead of the previous reputation he had. Only knowledgeable members can see that fluffy rank.
Your knowledge has not been shown so you got ranked back to none (unknown).
These two things could have happened to you.
But please do not panic. Simply prove that you know something, and your situation might change.
Remember that nothing is permanent and is changeable, and keep doing your stuff.