After login successfully you have to store user detail in session like :
Session["UserID"]="YourData"
.
.
Session["UserEmailID"]="YourData"
Then after redirect to Response.Redirect("Post_training.aspx");
On Post_training page check below code on page load event :
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
Session["UserEmailID"]=""
Response.Redirect("Login.aspx");
}
}
Please accept as answer and vote if help to you.