If session is null while working with ASHX and you can't manage to access session state implement IReadOnlySessionState in your class:

C#

public class Sertifika : IHttpHandler, System.Web.SessionState.IReadOnlySessionState
{
     public void ProcessRequest (HttpContext context)
     {
           context.Response.ContentType = "text/plain";
           context.Response.Write(context.Session["isil"]);
     }

     public bool IsReusable
     {
       get
       {
           return false;
       }
     }
}

VB:

Public Class Sertifika
    Implements System.Web.IHttpHandler
    Implements SessionState.IReadOnlySessionState

    Sub ProcessRequest(ByVal context As HttpContext) Implements IHttpHandler.ProcessRequest

        context.Response.ContentType = "text/plain"
        context.Response.Write(context.Session("isil"))

     End Sub
     ReadOnly Property IsReusable() As Boolean Implements IHttpHandler.IsReusable
        Get
            Return False
        End Get
    End Property

End Class