Did you let it come back up to room temp and a rewhip would help
You may also need to add more milk if it's still too stiff after returning to room temp and rewhipping it.
I've done that many times....lol. Letting it sit on the counter for a little while so it's not soooo cold and then adding a little milk and re-whipping it always does the trick for me. If for some reason you add a little too much milk, just toss in a little more powdered sugar and mix until it's back to it's regular consistency. Good luck!
Oh gosh - never throw out good buttercream!! lol 
It can be such a timesaver. Even your leftovers...unless it's got lots of cake crumbs in it or it's just been hanging around too long & hasn't been used, it's still good! I especially hang on to any leftover white icing that I can because you can always use it to tint colors if you need a small batch. I think some people even freeze leftover buttercream!?! Seems like I've read about that here at CC in the past.
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