This is typically a temporary condition and may be a result of a traffic spike to the website you're trying to visit, or any number of related issues.
A webs browser goes through the following steps when trying to reach a website:
- Resolve the name of the website (example.com) into an IP address (192.168.1.1) by looking up entries in DNS (Domain Name Server) records.
- Access the website via a socket connection (port) on that IP address (192.168.1.1:80 = socket 80).
- Send and receive HTTP data through that socket. The browser communicated back to you status codes indicating the success or failure of accessing this particular website.
In the case of your HTTP Error 503 - Service unavailable error I'm afraid there's not a whole lot you can do, but wait and see if the website comes back online. If you have contact information, you may want to make them aware of this issue (although in most cases, they already are).
Good luck. Let us know how it goes!