Updated on Oct 13, 2021
This particular error message is caused due to enabled maintenance on your Magento installation. If you did not specifically set that, it may have been caused due to the recent installation of extensions on your website via the Magento Connect Manager.
The fix is easy, actually. You simply need to access the root directory of your Magento installation. You can do that via FTP or directly using the File Manager service within your cPanel account. Navigate to the directory of your Magento website and look for a maintenance.flag file in it. Remove the file and you should be able to access your website normally again.
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