Is my dedicated server down?

If you can't see your web site or connect to your server, it's logical to assume the server has crashed. But it's worth checking a few things before you think about rebooting it.

The most basic test is to run a ping test - i.e. 'ping mydomain.com' from the command line.

It's worth running this against both the server's IP address and your domain name. If the IP address responds to the ping request, but the domain name doesn't, you might have a DNS problem or you domain might have expired. It's a good idea to follow this up with a whois lookup if you get this result.

If you can ping the server, open a ssh session and try logging onto it. If this fails, the server may be too busy to process logins, or the ssh daemon might not be running. In either case, you might need to reboot the server at this point. If the login is denied, you might have been blocked by your server's firewall.

This last point is also something you can test using a proxy to test your web site. Site's such as www.megaproxy.com allow you to test your site from an external location, independent of your own ISP and network.
If you can view your site through a proxy, but not from your own PC, it's worth checking whether any of your local network settings have changed.

If not, then it's possible the IP address you're connecting from has been blocked by the server's firewall. Many firewalls - for example, APF using BFD (Brute Force Detection) - can be set to block IP addresses that are the source of too many failed logon attempts - whether that be from the web site, email, ssh, etc. If you're working from a shared office, where many other people might be connecting to your server to collect their mail, or use online services hosted on it, you can find your IP address being responsible for a lot of failed login attempts fairly quickly.

If this happens, unless you have access from a previously white listed location, is to contact your hosting provider and ask them to remove the block. It would also be wise to ask them to white list your office IP address at the same time, to prevent this happening again.

If your office IP address is static - i.e. it's leased to you by your ISP and doesn't change - you shouldn't have any further problems as a result of failed logins. If, however, your IP address is dynamic - i.e. refreshed from your ISP's address pool, and changes regularly - you will have to notify your hosting provider every time it changes to avoid being blocked.