Here are a couple of things can be done when your website is down.
1. Is your hosting account suspended?
Did you make your hosting payment on time? Did your website exceed the bandwidth limit? Is your site throttled for some unknown reason? Check your inbox and see if there is any account suspension notification.
2. Is faulty node causing the outage?
Make a Traceroute and Ping check to see if there is a faulty node somewhere in your connection. If so, there is nothing much you can do but wait.
3. Is your site down for everyone?
Visit DownForEveryoneOrJustMe.com (awesome web tool) and check if your ISP is having connection problem with your name server. If so, contact your ISP to rectify the problem.
4. Is your site down because of planned maintenance?
Check with your web host server status report and see if there is any maintenance outage announcement. If you are using Bluehost or Hostmonster, you can visit serverstatus.bluehost.com/ or serverstatus.hostmonster.com/, see if your server is affected by regular maintenance.
What if none of the above is the cause?
If all of the above do not resolve your problem, then things are not looking good for you. Your web server outage is most probably an unplanned event and it is time to get in touch with your web host supports. Shoot an email or get on to the live chat system and communicate with the support staffs.