So I've used marshmallow fondant for several cakes, and never had this happen before! Making a cake for a friends baby shower, and she had to change the date due to rain. I just popped the cake covered in buttercream into the freezer for the week. Well I pulled the cake out of the freezer this afternoon, did a second layer of buttercream over my crumb coat, and applied my marshmallow fondant like normal. 

I move on to making my decorations and when I turn to look at my cake a half hour later the entire cake is shiny and appears to be melting!! Is it just the condensation from being frozen? I froze my last cake and didn't have this happen so I'm at a complete loss of what to do here. Her shower is tomorrow afternoon so if I have to scrap this one and start over I need to start NOW. 

Will it sort itself out once it reaches room temperature?? Should I douse the thing in more powdered sugar?? Please help!

It was a long time ago but I think I froze a test cake but I think I had fondant on it when I froze it. I do remember the condensation disappearing after a while but it did leave little shiny areas where the water had pooled on the fondant. I would have still been usable. However, it doesn't seem as if anybody knows the answer to your question right at this moment. Maybe you can start to bake a new cakes and if after a while it is still okay, you will know that the new one can be frozen and you will have a baked and frosted cake on hand... Unless it is a very specific cake... then I don't know what to tell you. I am sorry! I know how stressful this kind of thing is.

Thank you Kellig! I looked at a few posts about regular fondant sweating, and tried some of those fixes. I went after it with my hair dryer, and kept the excess that seemed more runny from completely melting by pushing it up with sharp knife while blow drying the area. Once the worst of the sheen was gone, I dusted it with some powdered sugar to help absorb any excess. I'm worried about the consistency of the the icing at this point, but I'm leaving it alone until the morning and I'll see how it seems then.