How can I reference simple DLL which contains one class but has no namespace assigned to it in an .aspx page

You don’t need a namespace to reference it; anything in the bin folder is implicitly in the same namespace as the page being compiled. So if your class looks like this:

Public Class MyClass1
Public Sub New()
End Sub
 
public Function getTheName(byval strval as string) as string
Return 'Hello' + strval
End function
End Class

VB.NET

Dim mc As New MyClass1
Response.Write(mc.getTheName)

C#

MyClass1 mc = new MyClass1();
Response.Write(mc.getTheName);