You should be able to re-download the Cron folder from https://download.whmcs.com/
Just select your current version and download the zip file, after you should be able to recopy the cron folder back over. If you move to a custom location, you will need to rename and ensure that config.php is setup correctly and the correct configuration is placed in the configuration.php file.
We recommend moving the Cron directory to a non-public directory above your web root to prevent web access. More details can be found here: