The most common cause for the 503 error is when your hosting account consumes all of the available CPU resources. You can check the resource usage on your server from your User Area -> My Accounts -> if you have more than one hosting account click on Manage next to it -> Information & Settings -> Usage Stats.
Your server resources can be exceeded due to spike in the traffic coming to your websites, poorly optimized website and database. Below you can find our WordPress and Joomla! optimization tutorials that will help you to reduce the resource usage of these applications:
Joomla! Performance Optimization
In case you are hosting your website on one of the Cloud hosting solutions, you can also enable the Autoscale feature from your User Area. This is a great way to avoid any downtime in times of unexpected traffic spikes. More on the Autoscale can be found in our article:
How to set up a cloud server to autoscale?
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destination source:https://www.siteground.com/kb/error_503_service_temporarily_unavailable/