I assume "third party" means non-employee. If that's correct, what is this person's relationship to your business? Also, are you the owner of the business or simply a management person with the business?
Who are "they," and what does "added" mean in this context? There are at least two other places in your post where you used the pronoun "they" without a clear antecedent.
Locked out and banned by whom?
Sure. Among other things, you can sue whomever it is that has taken possession of your personal property and seek an injunction requiring that whoever has your stuff release it to you.
I agree that consulting with a local attorney would be a good idea.

