I work on a existing ASP.NET MVC project. The solution uses the FluentNHibernate component(v 1.0.0).
Someone in the former team (solution was MVC2) that worked on it mod
If your dll has be created in .NET langage (C# / Vb.NET), it can be decompiled with some tools, like JetBrains dotPeek or Telerik JustDecompile. These tools can make all C# files and project file from a dll (or a exe).
Be carefull if your dll/exe has been obfuscated, the result code will be hardly readable (you'll have to use rename a lot).
In dotPeek and JustDecompile, right click on loaded dll, and choose "Create Project" (or "Export to project"). You'll get all C# files (or vb.net) and project file.
In some case, one of these tools will not be able to decompile. You can test another tools, it can work ...