how to redirect user to login page ?

Question

How to redirect a user to login page if he directly clicks on forgot password link, at this time the sesson is not null cause it has set to 18 minutes , so if he comes before 18 mins and directly clicks on forgout password i have to redirect him to Login page even if session is not null.

private void btnForgotPsd_Click()
{
  Response.Redirect("ForgotPsd.aspx?username"+txtUserName.Text)
}