1.document everything
2.get legal consultation
3.tell them that you are innocent and blaablaa, whatever that one guy said about severance etc.
4.get info from the registrar _when_ it was registered and how it was paid
4.1 prove that you were with someone or somehow that you could have not registered it
4.2 or prove that you could have not paid it (reasonably, ie. it doesnt show on your credit card.)

You will not get your job back, but you might get a better severance, and besides you really dont want to work in that company anyways.