Insert query not working in JAVA

String sql;
           //String temp = jComboBox1.getSelectedItem();
           sql = "Insert into mytable (s_id,fname,lname,enrolled) values(?,?,?,?)";
           ps.setString(1,t_id.getText());
           ps.setString(2,t_fname.getText());
           ps.setString(3,t_lname.getText());
           ps.setString(4,(String) jComboBox1.getSelectedItem());

           ps = con.prepareStatement(sql);
           ps.execute();
           JOptionPane.showMessageDialog(null, "Record Inserted");

           update_table();
        }
        catch(SQLException er){
            System.out.println(er);
            JOptionPane.showMessageDialog(null,er);
        }