Is it just you, or is it actually down?
Check isup.me to see if it's an internal network problem or your website is actually having problems.
Check the server is up and the DNS responds
$ ping xxx.xxx.xxx.xxx
$ host domain_name
Check that the application and Apache are running
Check the Apache logs and the application log
Are there error messages at the bottom of the file? Does anything look unusual?
tail -n 2000 /var/log/httpd/error_log
tail -n 2000 /var/log/httpd/access_log
tail -n 2000 APP_ROOT/log/production.log
Check that MySQL is running
Check the recent command line history
What was entered in the last few days? Could it have broken the site?