You can retrieve username by email like:
string username = Membership.GetUserNameByEmail(useremail)
You can use this now in PasswordRecovery.
Or using your own interface i.e. a Textbox and button do something like this:
protected void Button1_Click(object sender, EventArgs e)
{
string username = Membership.GetUserNameByEmail(txtEmail.Text);
MembershipUser mUser = Membership.GetUser(username);
string newpassword = mUser.ResetPassword();
//Send email using system.net.mail classes
}