The error message -
System.Net.Mail.SmtpException: The operation has timed out. at System.Net.Mail.SmtpClient.Send(MailMessage message) at MS_ContactUS.btnSend_Click(Object sender, EventArgs e) in c:\inetpub\wwwroot\MediaSign\MS-ContactUS.aspx.cs:line 48
clearly means that you are not able to connect to the SMTP server.
The reasons behind this can be:
1. Firewall has blocked access to it in your system.
2. If you are in a corporate network, then your network administrator has blocked access to gmail.
Check with your network administrator.