You can easily enable maintenance mode for your WordPress website using hPanel or plugin options:
Activating Maintenance mode using hPanel
Open Hosting → Manage:
Select WordPress Dashboard:
And enable Maintenance mode:
Your website will show a default maintenance message:
If you’d like to customize it, proceed with the plugins option below.
Activating Maintenance mode using plugins
If you would like to have a broader variety of options for your Maintenance page, you can install a specialized plugin. Simply access your Admin Dashboard (usually it’s available by domain.tld/wp-admin), hover the pointer over Plugins, then click on Add new:
Once there, search for a plugin called WP Maintenance Mode, install and activate it.
After the activation, configure the plugin’s settings and turn it on. To do this go to your left sidebar and choose Settings → WP Maintenance Mode:
Fill in all of your preferred details under the General tab. If you want to leave the default design, you can enable the maintenance mode right away:
If you would like to customize the page which visitors will be seeing (add the background image, colors, etc.), you can do this at the Design tab:
A comprehensive guide about Maintenance mode together with additional ways to put WordPress website into it can be found here: How to Put WordPress in Maintenance Mode