Try:
using (SqlConnection con = new SqlConnection(strConnect))
{
con.Open();
using (SqlCommand cmd = new SqlCommand("SELECT fileContent FROM myTable WHERE ID=@ID", con))
{
cmd.Parameters.AddWithValue("@ID", idOfFileInTable);
using (SqlDataReader reader = cmd.ExecuteReader())
{
if (reader.Read())
{
int id = (int) reader["iD"];
string content = (string) reader["fileContent"];
myTextBox.Text = content;
}
}
}
}