Here I have created sample that will help you out.
HTML
<div>
<div>
<cc1:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">
</cc1:ToolkitScriptManager>
<asp:Button ID="ShowPopup" runat="server" Text="Show Popup" OnClientClick="javascript:$find('ModalPopupExtender1').show();" />
<br />
<cc1:ModalPopupExtender ID="ModalPopupExtender1" runat="server" PopupControlID="PopupPanel"
TargetControlID="ShowPopup">
</cc1:ModalPopupExtender>
<asp:Panel ID="PopupPanel" runat="server" DefaultButton="ButtonOK" BorderColor="Black" BorderStyle="Solid"
BorderWidth="1">
<asp:UpdatePanel ID="UpdatePanel" runat="server" ChildrenAsTriggers="true">
<Triggers>
<asp:AsyncPostBackTrigger ControlID="ButtonOK" />
</Triggers>
<ContentTemplate>
<div class="popupheader">
Login Here
</div>
<br />
<br />
<div id="dvErrorMsg" runat="server" visible="false">
<asp:Label ID="lblMessage" runat="server"></asp:Label>
</div>
<table class="style1">
<tr>
<td>
<asp:Label ID="lblName" runat="server" Text="User Name"></asp:Label>
</td>
<td>
<asp:TextBox ID="txtName" runat="server" Width="180px"></asp:TextBox>
</td>
</tr>
<tr>
<td>
</td>
<td>
</td>
</tr>
<tr>
<td>
<asp:Label ID="lblPassword" runat="server" Text="Password"></asp:Label>
</td>
<td>
<asp:TextBox ID="txtPassword" runat="server" TextMode="Password" Width="180px"></asp:TextBox>
</td>
</tr>
<tr>
<td>
</td>
<td>
</td>
</tr>
<tr>
<td align="center" colspan="2">
<asp:Button ID="ButtonOK" runat="server" Text="OK" OnClick="Login" />
<asp:Button ID="ButtonCancel" OnClientClick="javascript:$find('ModalPopupExtender1').hide();"
runat="server" Text="Cancel" />
</td>
</tr>
</table>
</ContentTemplate>
</asp:UpdatePanel>
</asp:Panel>
</div>
</div>
Code
protected void Login(object sender, EventArgs e)
{
bool IsValidUser = false;
// Write Here Code to validate user.
if (!IsValidUser)
{
dvErrorMsg.Visible = true;
lblMessage.Text = "InValid Credential";
ModalPopupExtender1.Show();
}
else
{
// Write code to redirect on default page
//or if you have used form authentication then use below code
FormsAuthentication.RedirectFromLoginPage(txtName.Text, true);
}
}
for more information refer below link