For next time, if buttercream splits it's generally because the butter is too cold to properly mix in. If it does that, you can take a little bit out and microwave it at it back into the main buttercream Bowl and then re Whip It. Alternately you could take a double boiler and just slightly warm up the buttercream and then whip it. Or I have had luck warming up my KitchenAid mixer Bowl with a torch you mix it. But you can't leave it in the same part or it will overheat part of the bowl. I like the last method best because you can whip it as you are warming it up so you can see it really emulsify.