<form id="form1" runat="server">
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<asp:Button ID="btnShow" runat="server" Text="Show Modal Popup" />

<!-- ModalPopupExtender -->
<cc1:ModalPopupExtender ID="mp1" runat="server" PopupControlID="Panel1" TargetControlID="btnShow"
CancelControlID="btnClose" BackgroundCssClass="modalBackground">
</cc1:ModalPopupExtender>
<asp:Panel ID="Panel1" runat="server" CssClass="modalPopup" align="center" style = "display:none">
This is an ASP.Net AJAX ModalPopupExtender Example<br />
<asp:Button ID="btnClose" runat="server" Text="Close" />
</asp:Panel>
<!-- ModalPopupExtender -->
</form>

in code behind:
write:

if(textbox1.text!="YourCondition")
{
mp1.Show();
}
else
{
Response.Redirect("Form2.aspx");
}