Program does not contain a static ‘Main’ method suitable for an entry point

What do I do if I just want to create a project which contains a bunch of library functions? In other words no Main method is required. It seemed to be compiling a minute ago and then I added another .cs file and now I am confronted with this error message.

Create a .NET Class Library project if you only want a library project. If this is a project that already exists, you can set the Project Output type to a DLL ("Class Library") instead of an Executable ("Windows Application"/"Console Application") in the project properties.