Check if there is a file called maintenance.flag in the root folder of your Magento installation and if it is present delete it. You can look for the file and delete it using your File Manager from within your cPanel:
When Magento is performing certain tasks it temporarily creates this file. Magento checks for its existence and if it’s there will send users to the page you described.
It’s supposed to automatically delete this file when done processing whatever task it was doing, however there are occasions where something goes wrong and Magento failed to delete the file.