How to put my Joomla! website offline?
When making core changes to your Joomla!, it is a good idea to put your website offline to avoid any problems while making the changes.
Go to your Joomla! admin area > System > Global Configuration > Site tab. Use the Site Offline toggle and change the value to Yes. In the Offline Message box that will appear, modify the message which your visitors will see while your website is offline if needed. Click on Save to save the change and put your website offline.
Putting your website offline will not block access to your Joomla! admin area. To put your website back online, change the Site Offline value to No.