For backing up the contents of this blog I am using the excellent WordPress plugin called – you guessed it – BackupWordPress.
It offers the possibility to backup the database and the content files on the same server for free, and for 24$/year it sends your backup files to Dropbox. It works by using a number of utilities (zip for the files, mysqldump for the database) that are run in the command line using the PHP function shell_exec.
Recently I noticed that the plugin simply stopped working – without any visible error message. I realized it was because shell_exec was in the list of disabled PHP functions.
To find out if this function is disabled you can use phpinfo() or ini_get('disable_functions').
If indeed this function is disabled, then the plugin will not work at all, without returning any error.
If you have access to cpanel you can re-enable this function by going to Select PHP version / PHP Settings:

For more info about it, see the official WordPress plugin page.