Just guessing, but:
SqlCommand cmd = new SqlCommand(@"DELETE FROM [5th] WHERE ([5th]='" + textBox1.Text + "')", con);
Do you really have a column called
in your table called
I doubt it.
But more importantly, don't do any of that like that! Never 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.