Getting IErrorInfo.GetDescription failed with E_FAIL(0x80004005). public string generateInvoi...

   I want to retrieve last digits from the invoice number. I want to get value why I am getting this error. which is the keyword. I have tried [] also but it's giiving error.
public string generateInvoiceNo(string PRODUCTNAME)         {      
      try             {              
  con.Open();              
  string number;              
  OleDbDataAdapter DA1 = new OleDbDataAdapter
("SELECT Max(INVOICE_NO.SUBSTRING(9,5)+1) AS MAXINVOICE FROM PRODUCT_DETAILS", con);                
DataTable DT1 = new DataTable();              
  DA1.Fill(DT1);              
  if (DT1.Rows.Count > 0)                
{                     number = DT1.Rows[0]["MAXINVOICE"].ToString();                
}                
else            
    {                     number = "00001";                 }            
    OleDbCommand cmd2 = new OleDbCommand
("SELECT [D_NO].SUBSTRING(0,2) AS [D_NO] FROM [PRODUCT_MASTER] WHERE [PRODUCT_NAME]='" + PRODUCTNAME + "'", con);                
OleDbDataAdapter DA2 = new OleDbDataAdapter(cmd2);              
  DataTable DT2 = new DataTable();              
  DA2.Fill(DT2);                
string dno = DT2.Rows[0]["D_NO"].ToString() + System.DateTime.Today.ToString();                 return dno;             }        
    catch (Exception ex)        
    {                
MessageBox.Show(ex.Message);                
return null;            
 }          
  finally      
 {          
      con.Close();          
  }        
}