Cannot start more transactions on this session

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SQL-server-2000

SQL-Server

I am using sql server 2000 and my project is developed in vb6. and I am used ADODB connection. And My data is saved successfully and my project is running last 3 years without any error. But now, sometimes shown a error message during data save.
error message :
cannot start more transactions on this session
So could anybody help me why occurred this error.
My code sample is like

Db_SQL.BeginTrans
If Len(lblSerialNo.Caption) > 1 Then
       Db_SQL.Execute strSql   ''excute a store-procedure for insert data into mastertable
   
     For i = 1 To dataGrid.rows - 1
            Db_SQL.Execute strSql  ''excute a store-procedure for insert data into detailstable
     Next    
     Db_SQL.CommitTrans
  Else
        Dim adorstTemp As New ADODB.Recordset
        adorstTemp.CursorLocation = adUseClient
        adorstTemp.Open "select Slno from tblSLno",Db_SQL, adOpenStatic, adLockOptimistic, adCmdText
        mlngServiceSlno = CLng(adorstTemp("Slno ")) + 1
     Db_SQL.Execute strSql   ''Update data by StoreProcedure
     
      For i = 1 To dataGrid.rows - 1
            Db_SQL.Execute strSql  ''excute a store-procedure for insert data into detailstable
      Next    
     If Condition true then
       Db_SQL.Execute strSql
     end if 
        adorstTemp("Slno") = mlngServiceSlno
        adorstTemp.Update
        adorstTemp.Close
      
        Db_SQL.CommitTrans
  End if

Have any problem this code