Sudden 500 Internal Server Error nginx/1.0.11

[Googlemo]

Site was working fine for years... And suddenly I get this error. tried
reboot server - nothing, looking into logs and I see this kinds of errors:
connect() failed (110: Connection timed out) while connecting to upstream
writev() failed (110: Connection timed out) while sending request to
upstream
recv() failed (104: Connection reset by peer) while connecting to upstream

What happened..?

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,252546,252546#msg-252546