While there are many reasons why a site might return a 503 error code when it receives a request, one common one is that the site has a memory leak and has consumed all the available php memory. A quick remedy for this is to login to Control HQ, navigate to the site in question and open the Web Server section of the developer tools. From there press the "Restart PHP-FPM" button, which should get your site back in action quickly.

If the issue becomes a persistent problem the code and plugins of the website should be examined to determine where in the code the memory leak is occurring.