If you mean call a method in a .CS file from a ASPX.CS file, then just put the .CS file in your App_Code folder, and
refer to the static methods by classname:
string s = MyClass.StaticMethod();
Or create an instance of the class and use the methods on the instance:
MyClass inst = new MyClass();
string s = inst.Method();
If that isn't what you want to do, then please try to explain more clearly what you are trying to achieve.