Rectify this mapper error in C#

Dataaccesslayer.Mappername dows not contain a definition for 'Functionname' and no extension method 'Functionname' accepting a first argument of type Dataaccesslayer.Mappername could be found(are you using a using directive or an assembly reference)

public class Mappername
{
public void Functionname()
{}

}

I have this 'Functionname' in particular class
still strike with this error