Can't send email

All of a sudden, I can't send email. I can receive. Nothing has been
changed in "settings." I've been sending emails all day, then BAM! I get
this message. I've rebooted. What just happened?

The connection to the server has failed. Subject 'test', Account:
' (1)', Server: '',
Protocol: SMTP, Port: 25, Secure(SSL): No, Socket Error: 10060, Error Number: