Aside from exceptional situations (e.g., being declared a vexatious litigant) you can always file a lawsuit.

From what you've said, though -- and with the standard caution that I'm not a lawyer, this is not legal advice, and I'm sure I don't know all the details -- I think it would depend on what happened when "Person A & B decide[d] to start a company". If Person A can show that there was mutual intent to create an equal partnership, he would probably get something out of a lawsuit; a lack of paperwork does not invalidate an agreement (provided that Person A can show that such an agreement existed). It's even possible that a court would decide that Person B had acted to willfully defraud Person A by incorporating the company on his own.

Of course, all this would go out the window if Person B could show that the partnership between himself and Person A had been terminated (either by mutual agreement, or by abandonment).

Short answer: Go talk to a lawyer, and provide him with far more details than you've provided here.