Just declare your bindstate() method as a WebMethod like
[System.Web.Services.WebMethod]
public void bindstate(string id)
{
try
{
string connectionString = ConfigurationManager.AppSettings["ConnectionString"];
OracleConnection conn = new OracleConnection(connectionString);
conn.Open();
String Str = "select STATE_ID,STATE_NAME from com_state_master where COUNTRY_ID="+id;
OracleCommand cmd = new OracleCommand(Str, conn);
ddlState.DataSource = cmd.ExecuteReader();
ddlState.DataTextField = "COUNTRY_NAME";
ddlState.DataValueField = "COUNTRY_ID";
ddlState.DataBind();
conn.Close();
}
finally
{
}
}