The server is temporarily unable to service your request due to maintenance downtime or capacity problems

Your magento website might be in maintenance mode. You won’t be able to access the magento site if it is in maintenance mode.
FIX :

I saw the above error message while trying to update/install my magento website. The update failed and my site shows the above error when refreshed. Do the below steps to fix the error.

1. Log into your linux server via SSH as root

2. Go to var directory within your magento installation directory

cd /home/user/public_html/magento2/var

3. Remove maintenance.flag file in var directory. It is just an empty file.

rm -f .maintenance.flag

.maintenance.flag file might be either in magento installation directory or in var directory.

Try refreshing the site after removing the maintenance flag. Clear the magento cache if you’re still getting the same error on the browser.

You can delete the file either through FTP if you don’t have server root access OR you can delete the .maintenance.flag file through ‘file manager’ in cPanel.