Question: Is it possible to decompile a `.xnb` file? I've lost my shader code, but only have my `.xnb` file left.

Answer: 
It's unlikely you'll be able to retrieve the source from the xnb file as it has undergone compilation into a binary format during the XNA pipeline stages. While it's theoretically not impossible, in my opinion you'd probably be much better off rewriting the shader..
<http://mirror.dataorb.net/XNB_Exporter.zip> - Maybe this can help you.