Firstly, stop doing it like that!
Do not concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Use Parametrized queries instead:
Using con As New SqlConnection(strConnect)
con.Open()
Using da As New SqlDataAdapter("SELECT MyColumn1, MyColumn2 FROM myTable WHERE mySearchColumn = @SEARCH", con)
da.SelectCommand.Parameters.AddWithValue("@SEARCH", myTextBox.Text)
Dim dt As New DataTable()
da.Fill(dt)
...
End Using
End Using
Second, that is stuffed full of SUM operations, which will return null is there is any one null value in your column - so any single value of null will cause your sum to be zero.
Look at your data!