Fix a Prestashop 1.7 based website

Hi, I have a Prestashop website and it was working very well until a colleague has changed the PHP version from 7.1 to 7.3 and it all stopped working, we reverted back to 7.1 but still causing the admin panel not to be able to login as well as the front panel not loading any javascrips in the pages, it was perfectly working before that, we need somebody to fix both the admin panel as well as the Javascript, no backup to revert to.

My budget is 40$ for this project, needs to be able to work right away and fix it ASAP, it is a live website.