I used a chocolate buttercream recipe to ice a cake and now that it's crusted over, it won't smooth and isn't spreadable anymore at all. Also, the crust has little cracks in it...

How can I smooth my cake or get the icing soft enough again to spread?

Have you tried the Viva method yet? Get a Viva paper towel and lay it on your cake and use your hand or a spatula to light go over the surface of the cake. Smooths out like nothing else.
Good luck

Yes, I tried the viva papertowl...all it's doing is moving the icing and not smoothing anything out. It looks the same when you're done just looks a little shifted.

Either add another layer of icing or scrape it off and start over.

I usually can add another thin layer of icing. If it's crusted enough, and if the new coat of icing is thin and smooth enough, it all works out well.