function is not being called

<script type="text/javascript">
        var classid , examid;
        function GetData()
        {
    debugger
            classid = document.getElementById("<%=ddlbatch.ClientID%>").value;
            examid  = 0;
            // I need to read returned data table below
           GetData(classid,examid);
            //WebService.GetData(classid,examid);
        }

public DataTable GetData(int classid, int examid)
    {
        
        DataTable dt = new DataTable();
       
        cn = WebConfigurationManager.ConnectionStrings["ExpertsSchoolConnectionString"].ConnectionString;
        using (SqlConnection con = new SqlConnection(cn))
        {
            SqlCommand com = new SqlCommand();
            //  com.Connection = con;
            com.CommandType = CommandType.StoredProcedure;
            com.CommandText = "ES_SUBJECT_EXAM";
            SqlParameter classid1 = new SqlParameter("@classid", SqlDbType.Int);
             classid1.Value = classid ;
            com.Parameters.Add(classid1);

            SqlParameter examid1 = new SqlParameter("@examid", SqlDbType.Int);
            examid1.Value = examid ;
            com.Parameters.Add(examid1);
            try
            {
                con.Open();
                SqlDataAdapter da = new SqlDataAdapter();
                da.SelectCommand = com;
                da.Fill(dt);
            }
            catch (Exception ex)
            {
                if (con.State == ConnectionState.Open)
                {
                    con.Close();
                }
                dt = null;
            }
            finally
            {
                con.Close();
            }
            return dt;
        }
    }