Firstly you create session for email id which is unique and on submit button fetch data match data if email id and password is valid then Response.Redirect("~/Mainpage.aspx");
For Example
This is for login page submit button click event
protected void submit_Click(object sender, EventArgs e)
{
Session["emailId"] = txtemailId.Text;
Response.Redirect("~/MainPage.aspx");
}
This for MainPage to Show Email Id
protected void Page_Load(object sender, EventArgs e)
{
lblEmailId.Text = Session["emailId"].ToString();
}
try this it should work.