WordPress sites can go into Maintenance mode unexpectedly for a number of reasons. One of the major cause is due to incomplete upgrade caused by connection timeout.
WordPress upgrades are usually attended. As such, user would know if the upgrade halted halfway.
However, with WordPress Toolkit or WordPress Manager for Softaculous, WordPress upgrades are usually set to automatic for Minor upgrades and runs in the background without user provision which can lead to the site entering maintenance mode.
How to solve this?
WordPress maintenance are activated by creating a file name .maintenance in the WordPress site document root. Follow the steps below to reactivate the site.
To prevent such unexpected maintenance mode, turn off auto upgrade entirely.
To do so:-
1. Go to WordPress Toolkit. Steps on cPanel and Plesk defers as below.
a) cPanel - Accessing cPanel WordPress Toolkit
b) Plesk - How to use WordPress Toolkit in Plesk
2. Click Autoupdate Settings.
3. Select 'No' for all 3 options.