Use the following to redirect to login page in DotNetNuke:

If Not Request.IsAuthenticated Then

    Response.Redirect(Globals.NavigateURL(PortalSettings.LoginTabId, True, PortalSettings, “Login”, “returnurl=” & Server.UrlEncode(Request.Url.ToString)))

End If

Courtesy: http://blog.pnbconsulting.com.au/index.php/dotnetnuke-redirect-after-login/