You may have a case against them for malpractice, in which case, you could sue for all your costs they caused you to incur. That would not include taxes you should have paid anyway, but could include interest or penalties on it, plus any professional (e.g. lawyer) expenses you've incurred to clean up the mess. You may wish to consult with a local attorney, with whom you can share all the inforamtion in detail and confidence, so you can get a more definitive answer, but it sounds like it is worthwhile at least looking into a lawsuit.