"Service Unavailable," or 503, errors are a common problem on overloaded websites. This error can best be thought of as a down-for-maintenance or even a closed-for-repairs sign since a 503 message is essentially the Web server saying it is not currently accepting requests. The message is often sent without an explanation but could be caused by a number of things.
Since the Web server is still able to send an error code, you can assume that the website's administrator is aware of the problem and is attempting to resolve it. The 503 codes are a service interruption, and most responsible administrators set up their Web servers to notify them when something goes terribly wrong. Oftentimes, a 503 error designates that the website has been purposefully taken offline while someone attempts to resolve an issue.
Too Many Requests
Another problem can come from too many requests to the website. Many websites are incapable of handling large amounts of traffic, which can occur if they become part of a news story or are posted on a popular blog or website. In fact, most Web servers are hosted on extraordinarily low-spec virtual machines that can only handle small volumes of traffic at any one time.
Frequently, hacker groups use denial-of-service attacks to protest censorship or copyrights. Denial-of-service attacks send thousands of requests per second to a page, keeping the page from functioning. This can best be thought of as someone blocking the entrance to a building through a sit-in. The server is simply full and unable to respond with anything other than a 503 message.
Additionally, if the server is running a complex Web-based application, the server may have trouble processing the requests of only a few users due to the processing power required. This can cause your requests to be timed out and the server to respond with the 503 error message due to its inability to serve you the proper page.