dropdown menu evenot working in aspx page

My dropdown list event not working. bellow my aspx code. 

<%@ Page Title="" Language="VB" MasterPageFile="~/MasterPage.master" AutoEventWireup="false" CodeFile="Employee.aspx.vb" Inherits="Employee" EnableViewState ="false" %>

<asp:Content ID="Content1" ContentPlaceHolderID="head" Runat="Server">
<style type="text/css">
.style24
{
width: 13px;
}
.style25
{
}
.style26
{}
.style28
{
width: 117px;
height: 23px;
}
.style29
{
height: 23px;
}
.style30
{
width: 117px;
height: 24px;
}
.style31
{
height: 24px;
}
.style34
{
width: 117px;
height: 19px;
}
.style35
{
height: 19px;
}
.style36
{
width: 117px;
height: 22px;
}
.style37
{
height: 22px;
}
.style38
{
width: 270px;
height: 227px;
}
.style45
{
width: 270px;
}
.style46
{
width: 270px;
height: 70px;
}
</style>
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
<p>
<br />
</p>
<p>
<table style="width: 103%;">
<tr>
<td align="center" class="style29" colspan="3">
<asp:Label ID="Label1" runat="server" Font-Bold="True" Text="Welcome "
Font-Underline="True"></asp:Label>
<asp:Label ID="lblEmpName" runat="server" Font-Bold="True"
Font-Underline="True"></asp:Label>
</td>
</tr>
<tr>
<td class="style38">
<%-- <asp:Panel ID="Panel1" runat="server" BackColor="White"
Height="143px" style="margin-top: 0px" BorderColor="White">--%>
<table style="width:100%; height: 142px;" bgcolor="Silver">
<tr>
<td align="center" bgcolor="White" class="style26" colspan="2">
<asp:Label ID="Label2" runat="server" Font-Bold="True" Font-Underline="True"
Text="Employee Profile"></asp:Label>
</td>
</tr>
<tr>
<td align="right" bgcolor="#CCCCCC" class="style28">
<asp:Label ID="Label3" runat="server" Text="Employee ID: "></asp:Label>
</td>
<td bgcolor="#CCCCCC" class="style29">
<asp:Label ID="lblEmpID" runat="server" Text="Label"></asp:Label>
</td>
</tr>
<tr>
<td align="right" bgcolor="White" class="style30">
<asp:Label ID="Label4" runat="server" Text="Employee Name: "></asp:Label>
</td>
<td bgcolor="White" class="style31">
<asp:Label ID="lblName" runat="server" Text="Label"></asp:Label>
</td>
</tr>
<tr>
<td align="right" bgcolor="#CCCCCC" class="style34">
<asp:Label ID="Label5" runat="server" Text="Designation: "></asp:Label>
</td>
<td bgcolor="#CCCCCC" class="style35">
<asp:Label ID="lblDesignation" runat="server" Text="Label"></asp:Label>
</td>
</tr>
<tr bgcolor="White">
<td align="right" bgcolor="White" class="style36">
<asp:Label ID="Label6" runat="server" Text="Employee Group: "></asp:Label>
</td>
<td bgcolor="White" class="style37">
<asp:Label ID="lblGroup" runat="server" Text="Label"></asp:Label>
</td>
</tr>
</table>
<%-- </asp:Panel>--%>
</td>
<td class="style24" rowspan="2">
&nbsp;</td>
<td align="center" rowspan="2" valign="top">
<br />
<br />
<br />
<asp:Label ID="Label8" runat="server" Font-Bold="True" Font-Underline="True"
Text="Pending Requests"></asp:Label>
<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="true">
<asp:ListItem>aa</asp:ListItem>
<asp:ListItem>bb</asp:ListItem>
</asp:DropDownList>
<asp:DropDownList ID="ddlRequest" runat="server" CausesValidation="true">
<asp:ListItem Selected="True">Leave Request</asp:ListItem>
<asp:ListItem>Loan Request</asp:ListItem>
<asp:ListItem>Travel Request</asp:ListItem>
</asp:DropDownList>
<br />
<br />
<asp:GridView ID="dgvPendingReq" runat="server"
BackColor="White" BorderColor="#999999" BorderStyle="Solid" BorderWidth="1px"
CellPadding="3" ForeColor="Black" GridLines="Vertical" Height="72px"
Width="495px" style="margin-left: 0px">
<AlternatingRowStyle BackColor="#CCCCCC" />
<FooterStyle BackColor="#CCCCCC" />
<HeaderStyle BackColor="Silver" Font-Bold="True" ForeColor="Black"
Height="10px" />
<PagerStyle BackColor="#999999" ForeColor="Black" HorizontalAlign="Center" />
<SelectedRowStyle BackColor="#000099" Font-Bold="True" ForeColor="White" />
<SortedAscendingCellStyle BackColor="#F1F1F1" />
<SortedAscendingHeaderStyle BackColor="Gray" />
<SortedDescendingCellStyle BackColor="#CAC9C9" />
<SortedDescendingHeaderStyle BackColor="#383838" />
</asp:GridView>
<br />
<br />
<asp:Label ID="Label9" runat="server" Font-Bold="True" Font-Underline="True"
Text="Approved Requests"></asp:Label>
<asp:DropDownList ID="ddlAppRequest" runat="server" AutoPostBack="True">
<asp:ListItem Selected="True">Leave Request</asp:ListItem>
<asp:ListItem>Loan Request</asp:ListItem>
<asp:ListItem>Travel Request</asp:ListItem>
</asp:DropDownList>
</td>
</tr>
<tr>
<td align="center" class="style46">
<br />
<br />
<asp:Label ID="Label7" runat="server" Font-Bold="True" Font-Underline="True"
Text="Requests"></asp:Label>
</td>
</tr>
<tr>
<td align="center" bgcolor="#CCCCCC" class="style45">
<asp:HyperLink ID="HyperLink8" runat="server" ForeColor="Black"
NavigateUrl="~/Leave.aspx" ToolTip="Leave Request">Leave Request</asp:HyperLink>
</td>
<td class="style24" rowspan="3">
</td>
<td align="center" rowspan="3">
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
<asp:GridView ID="dgvApprovedReq" runat="server" AutoGenerateColumns="False"
BackColor="White" BorderColor="#999999" BorderStyle="Solid" BorderWidth="1px"
CellPadding="3" ForeColor="Black" GridLines="Vertical" Height="123px"
Width="409px">
<AlternatingRowStyle BackColor="#CCCCCC" />
<Columns>
<asp:BoundField HeaderText="Type or Request" />
<asp:BoundField HeaderText="Request ID" />
<asp:BoundField HeaderText="Approved Date" />
</Columns>
<FooterStyle BackColor="#CCCCCC" />
<HeaderStyle BackColor="Silver" Font-Bold="True" ForeColor="Black"
Height="10px" />
<PagerStyle BackColor="#999999" ForeColor="Black" HorizontalAlign="Center" />
<SelectedRowStyle BackColor="#000099" Font-Bold="True" ForeColor="White" />
<SortedAscendingCellStyle BackColor="#F1F1F1" />
<SortedAscendingHeaderStyle BackColor="Gray" />
<SortedDescendingCellStyle BackColor="#CAC9C9" />
<SortedDescendingHeaderStyle BackColor="#383838" />
</asp:GridView>
</td>
</tr>
<tr>
<td align="center" bgcolor="#CCCCCC" class="style45">
<asp:HyperLink ID="HyperLink9" runat="server" ForeColor="Black"
NavigateUrl="~/Loan.aspx" ToolTip="Loan Request">Loan Request</asp:HyperLink>
</td>
</tr>
<tr>
<td align="center" bgcolor="#CCCCCC" class="style45">
<asp:HyperLink ID="HyperLink10" runat="server" ForeColor="Black"
NavigateUrl="~/Travel.aspx" ToolTip="Travel Request">Travel Request</asp:HyperLink>
</td>
</tr>
<tr>
<td class="style25">
&nbsp;</td>
<td class="style24">
&nbsp;</td>
<td>
&nbsp;</td>
</tr>
</table>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<asp:DropDownList id="abc" runat="server" AutoPostBack="true" OnSelectedIndexChanged="abc_SelectedIndexChanged">
<asp:ListItem>asd</asp:ListItem>
<asp:ListItem>vcf</asp:ListItem>



</asp:DropDownList>
</asp:Content>