[Solved] ASP.Net Validator Error Messages hides on PostBack

By default this will not happen and you need to do the following trick i.e. make the validators validate On PostBack

HTML

<asp:DropDownList runat="server" AutoPostBack = "true">
    <asp:ListItem Text="text1" />
    <asp:ListItem Text="text2" />
</asp:DropDownList>
<asp:TextBox ID = "txtName" runat="server" />  
<asp:RequiredFieldValidator ErrorMessage="Required" ControlToValidate="txtName"
    runat="server" ValidationGroup = "registration"/>
    <br />
<asp:Button Text="Submit" runat="server" ValidationGroup = "registration" />

Code

protected void Page_Load(object sender, EventArgs e)
{
    if (this.IsPostBack)
    {
        ClientScript.RegisterClientScriptBlock(this.GetType(), "Validation", "window.onload = function() { Page_ClientValidate('registration'); };", true);
    }
}