Hi Mehram,

Please refer below sample code. You need to call the change function inside JavaScript windows.onload event.

HTML

<asp:DropDownList ID="cboVoucherType" runat="server" onchange="change();">
    <asp:ListItem Text="Single" Value="0" />
    <asp:ListItem Text="Multiple" Value="1" />
</asp:DropDownList>
<div id="divEntrySingle">
    Single
</div>
<div id="divEntryMultipal">
    Multiple
</div>

<script type="text/javascript">
    window.onload = function () {
        change();
    }
    function change() {
        var DropdownList = document.getElementById('<%=cboVoucherType.ClientID%>');
        var SelectedIndex = DropdownList.selectedIndex;
        var divEntrySingle = document.getElementById('divEntrySingle');
        var divEntryMultipal = document.getElementById('divEntryMultipal');
        if (SelectedIndex == 1) {
            divEntrySingle.style.display = "none";
            divEntryMultipal.style.display = "block";
        }
        else {
            divEntrySingle.style.display = "block";
            divEntryMultipal.style.display = "none";
        }
    }
</script>

Screenshot