Hi!! I've made buttercream only a billion times and never had a problem. My recipe is as follows:
600g icing sugar
dash of milk or cream
It's always come together beautifully, and I always beat my butter for a good 5 mins before I add any icing sugar. Today it looks curdled and grainy. It's only coloured with a few drops of Americolor gel, so there's definitely not too much liquid in it. I've just put it back into my mixer with the whisk attachment to see if it gets better. I need to pipe it onto my cupcakes in the next half hour for a school fundraiser!! Any quick fixes????