First off, don't panic. 9 times out of 10, it is the internet somewhere between you and the server that is experiencing problems. Below lists two options that you can use to see if it is your website that is down OR if it is down somewhere between you and your website.
1: Try performing a Ping Test
2: Try performing a Traceroute
Try #1 (Ping Test) and if your site is NOT reachable, try #2 (Traceroute)