Download XBMC again, and ftp the "skins" folder back into your Xbox.
If XBMC was your default dash, place the "skins" folder inside of "C". I'm imagining you will need to be using an X3 chip, or a boot disk in order to successfully FTP. If need be, you can get an EvolutionX boot disk from the usual places. It's good to keep one for situations like this. Worse come to worse, COMPLETELY reinstall XBMC.
Let me know what happens.