Ask your IT to give you PhpMyAdmin ot similar GUI interface to access your Database and run this line:

UPDATE mdl_user SET `maildigest` = REPLACE(`maildigest`,"0","1")

You can choose changes from 0 'no' to 1 'full' or 2 'Subject'

If you are Admin of your Moodle, you can install 

the plugin 'Adminer' https://moodle.org/plugins/view.php?plugin=local_adminer