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Hi,
from the error it seems that you need to do 2 things:
Create the folder /app/cache/, as it does not exist and cannot be emptied"
Also you/or your hosting should switch the PHP to version version 7.1.3 or higher, here you have the PHP compatibility chart: https://devdocs.prestashop.com/1.7/basics/installation/system-requirements/#php-requirements
About the site restore back to the old version you should ask the hosting as they usually have backups of files and database back to a couple of days , and they can restore it, also if you have the files nd database these can be restored also to the old version.
Kind regards, Leo