After almost five years of happy marriage, immigration is accusing my wife of marriage fraud

Could you please give more information? Did anyone come to talk to you from immigrations or is it something you got in the mail? How are they acussing her of this? What were the circumstances of her first marriage, please give us as much info as you know so we can advise you better. Are you a US citizen?
However my best advice for now is NOT to talk to anyone about this without an attorney present. Please provide more information, so we can help.

Everything depends on the facts and circumstances of your situation. You really need to take all of your paper work, from both marriages and all immigration applications to an immigration attorney in your area.

That is very unusual - you should consult with an immigration attorney instead of trying to second guess the situation.