In order to help you with this we need to know what kind of frosting it is. Is it just butter cream, or is it a cooked frosting? I can't imagine how buttercream could get grainy, and if it is a cooked frosting, like seven minute or boiled frosting, than I think it would have been cooked too long, and that there would be nothing to do but start over or serve it grainy.
Login using the form on the top of the page to post feedback (if you are a registered user). If you have not yet registered, click here to do so. It's FREE!.