revert website after fault

Hi
I was working on wordpress (ssaff.org.au) and made a change to function.php, after which the website is totally blank
i would like to revert the changes i made but am unfamiliar with tft, can you please direct me on how to undo the changes made or revert the website to a previous day/backup
thank you

We do not know if WordPress does backups and how/where. So we cannot help you with that. Please ask their support.