The problem in your code is you adding combobox after setting frame's visibily=true. Please add all the components before setting frame's visibilty. I changed your constructor. Please note that using a null layout is not a good habbit (I think so) use any of LayoutManagers
DisplayEmpData() {
l0 = new JLabel("Fatching Employee Information");
l0.setForeground(Color.red);
l0.setFont(new Font("Serif", Font.BOLD, 20));
l1 = new JLabel("Select name");
b1 = new JButton("submit");
l0.setBounds(100, 50, 350, 40);
l1.setBounds(75, 110, 75, 20);
b1.setBounds(150, 150, 150, 20);
b1.addActionListener(this);
setTitle("Fetching Student Info From DataBase");
setLayout(null);
setSize(500, 500);
setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
add(l0);
add(l1);;
add(b1);
try {
Vector v = new Vector();
String url="jdbc:odbc:Driver={Microsoft Access Driver (*.mdb, *.accdb)};DBQ=" + "C:\\users\\ppreeti\\emp_details.accdb";
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
con=DriverManager.getConnection(url,"","");
/*con = DriverManager.getConnection("jdbc:oracle:thin:@mcndesktop07:1521:xe", "sandeep", "welcome");*/
st = con.createStatement();
rs = st.executeQuery("select uname from emp");
while (rs.next()) {
ids = rs.getString(1);
v.add(ids);
}
c1 = new JComboBox(v);
c1.setBounds(150, 110, 150, 20);
add(c1);
st.close();
rs.close();
} catch (Exception e) {
}
setVisible(true);
}