Rescuing peanut butter frosting
There is no way to rescue your frosting. It is not just a mixture of ingredients, it used to have a very complicated internal structure, which is now gone. You will have to start a new batch. 

I assume that the taste of the frosting hasn't changed much. It cannot be used for its original purpose, but you should be able to repurpose it in many ways, for example as a bread spread, or as a filling somewhere.