If the library is internal (written by the company), then the company should keep the source code somewhere, probably in a different DVCS/VCS repository.
In that case just ask a colleague where the source code is kept.
If the library is external, it is only possible to see the source code if the project is open-source, in which case just search for the project.
If the library is internal but the company does not have the source code (due to a technical disaster, server loss, etc.), then tell your superior that you cannot progress in the project without the source code of that library, and it will help you as power.
Libraries (dlls) do not have a copy of the source code. They are written in a different language, IL (Intermediate Language), which is below C# and levels above machine language.
There are some projects that try to convert IL to C# (Jetbrains dotPeek) code, but the result is not 100% accurate, and the original code comments are lost. These tools are mostly used for academic purposes (learn what’s going on Behind the Scenes), almost never for professional purposes, and certainly your boss/manager should not put you in a position where you need to use these tools to do your job.
That library and' internal (was written by the company) or external?
– dcastro
It was written by the company, it’s internal, it’s ours.
– pnet