Here are the most common ways you might see the "service unavailable" error:
503 Service Unavailable
503 Service Temporarily Unavailable
Http/1.1 Service Unavailable
HTTP Server Error 503
Service Unavailable - DNS Failure
HTTP Error 503
Error 503 Service Unavailable
503 Service Unavailable errors can appear in any browser in any operating system, including Windows 10 back through Windows XP, macOS, Linux, etc.... even your smartphone or other nontraditional computers. If it has internet access, then you could see a 503 in certain situations.
The 503 Service Unavailable error displays inside the browser window, just as web pages do.
Cause of 503 Service Unavailable Errors
Most of the time, a 503 error occurs because the server is too busy or because there's maintenance being performed on it.
Note: Sites that use Microsoft IIS may provide more specific information about the cause of a 503 Service Unavailable error by suffixing a number after the 503, as in HTTP Error 503.2 - Service Unavailable, which means Concurrent request limit exceeded.
How To Fix the 503 Service Unavailable Error
The 503 Service Unavailable error is a server-side error, meaning the problem is usually with the web site's server. It's possible that your computer is having some kind of problem that's causing the 503 error but it's not likely.
Regardless, there are a few things you can try:
Retry the URL from the address bar again by clicking the reload/refresh button or pressing F5.
Even though the 503 Service Unavailable error means that there's an error on another computer, the issue is probably only temporary. Sometimes just trying the page again will work.
Important: If the 503 Service Unavailable error message appears while paying for an online purchase, be aware that multiple attempts to checkout may end up creating multiple orders - and multiple charges! Most payment systems, and some credit card companies, have protections from this kind of thing but it's still something to be aware of.
Restart your router and modem, and then your computer or device, especially if you're seeing the "Service Unavailable - DNS Failure" error.
While the 503 error is still most likely the fault of the website you're visiting, it's possible that there's an issue with the DNS server configurations on your router or computer, which a simple restart of both might correct.
Tip: If resetting your equipment didn't correct the 503 DNS Failure error, there may be temporary issues with the DNS servers themselves. In this case, pick new DNS servers from my Free & Public DNS Servers List and change them on your computer or router. See How To Change DNS Servers if you need help.
Another option is to contact the website directly for help. There's a good chance that the site's administrators already know about the 503 error but letting them know, or checking the status on the problem, isn't a bad idea.
See my Website Contact Information list for contact information for popular websites. Most sites have support-based social network accounts and some even have phone numbers and email addresses.
Tip: If you the site 503 error is a popular one and you think it might be down completely, a smart Twitter search can usually give you the answer. Try searching for #websitedown on Twitter, replacing the website with the site name, as in #facebookdown or #youtubedown. An outage on a big site will usually generate lots of talk on Twitter.
Come back later. Since the 503 Service Unavailable error is a common error message on very popular websites when a huge increase in traffic by visitors (that's you!) is overwhelming the servers, simply waiting it out is often your best bet.
Frankly, this is the most likely "fix" for a 503 error. As more and more visitors leave the website, the chances of a successful page load for you increases.
Fixing 503 Errors on Your Own Site
With so many different web server options out there, and even more general reasons why your service might be unavailable, there isn't a straightforward "thing to go do" if your site is giving your users a 503.
That said, there are certainly some places to start looking for a problem... and then hopefully a solution.
Start by taking the message literally - has something crashed? Restart running processes and see if that helps.
Beyond that, look at not-so-obvious places where something may have hiccuped. Where applicable, look at things like connection limits, bandwidth throttling, overall system resources, fail-safes that may have triggered, etc.