Yes, that's right, buttercream can break! This is often a temperature issue. If your butter is too cold, you can let the bowl sit in a warm (not hot) water bath and beat with a hand mixer until the ingredients come together. If the butter is too warm, let the bowl sit in the freezer for a few minutes, and then try to mix the ingredients again. Maybe the entire batch is broken and perfectly room temperature, you can try this pro baker trick: scoop some of the broken buttercream from the mixture and microwave it just until it melts. Add it back to the bowl and beat until everything comes together.