calling a C # function even when the page is closed.

Do you have your SMTP code ready for sending email ? if NO then visit my this answer .

Before that do these:
1. Create a console application.
2. Write a code to fetch list of emails of all the user which are suppose to recieve those email. (in this you can provide filteration od users.)
3. Add this code to send email

protected void SendMail()
{
    // Gmail Address from where you send the mail
    var fromAddress = "Gmail@gmail.com";
    // any address where the email will be sending
    var toAddress = YourEmail.Text.ToString(); 
    //Password of your gmail address
    const string fromPassword = "Password";
     // Passing the values and make a email formate to display
    string subject = YourSubject.Text.ToString();
    string body = "From: " + YourName.Text + "\n";
    body += "Email: " + YourEmail.Text + "\n";
    body += "Subject: " + YourSubject.Text + "\n";
    body += "Question: \n" + Comments.Text + "\n";
    // smtp settings
    var smtp = new System.Net.Mail.SmtpClient();
    {
        smtp.Host = "smtp.gmail.com"; // provide your smtpserver
        smtp.Port = 587;
        smtp.EnableSsl = true;
        smtp.DeliveryMethod = System.Net.Mail.SmtpDeliveryMethod.Network;
// provide your stmp server credentials
        smtp.Credentials = new NetworkCredential(fromAddress, fromPassword);
        smtp.Timeout = 20000;
    }
    // Passing values to smtp object
    smtp.Send(fromAddress, toAddress, subject, body);
}

4. Test it throughly.
5. build it in release mode.
6. Take the exe from your bin folder and add that into Schedule TASK of your server
7. If you dont know how to do it refer this : ..Schedule a Task[^]
8. Dont forget to mark it as answer if you have found it working for you