Question: What happened to my Swiss Butttercream ?

When I made my Swiss Butter cream it was perfect. I put it in the frig until the next day and when I tried to rewhip it, it seperated and became watery with curded butter? I could not use it. I have never had this problem before. It was perfect last night. I know it says you can freez it and then rewhip after it thaws. Can you not put it in the frig?