Service Temporarily Unavailable will be displayed on shared premium hosting if your account exceeds its 'Maximum Entry Processes' an entry process is a process your hosting account has created on the shared hosting server (eg running php to parse a webpage or cron jobs running in the 'background') .

To see your resource usage on entry processes (historic and current) . simply log into your control panel, then click on 'Resource Usage' then click on [Details] to see graphs of usage.

The best tip to reduce Entry process usage is to make your webpage(s) / scripts installed on your site return data quicker ( eg adding a caching plugin to the parts of your sites that are slowest ) , making a page use an entry process for a shorter amount of time allows more pages to be served per second without backlogging entry processes. 

You can also upgrade to a higher plan to get more processes to mitigate this and give your site more resources.

See here how to upgrade:

https://kb.ifastnet.com/index.php?/article/AA-00245/0/How-to-upgrade-cPanel-premium-hosting-to-a-better-plan-like-VPS-or-business-hosting.html

And don't hesitate to contact support to discuss this we are happy to check your site and offer solutions :

https://support.ifastnet.com/login.php