Okay, figured it out.
So the problem was the php-fpm service was causing the site to not load after the change. To fix this:
1) we first stopped the php-fpm service
2) the site loaded properly
3) relaunched the php-fpm service
4) regenerated the .htaccess and robots.txt file.
Also, after the relaunch, i lost all my product images. Tried regenerating the .htacces file in SEO URL, however it did not work.
To fix this, i had to rename the existing .htaccess file and then regenerate a new one from SEO URL.
Good to go.
ps: the prestashop version is 1.7.5