I suspect something might be wrong in your .htaccess for the rewrites, but it's an odd situation. If you can still login to the MODX manager, head to System (cog icon) > System Settings, find friendly_urls, and set it to 0. Clear the cache. Maybe you can browse the site after that?
Checking logs is also useful. Check your server/php logs, as well as the MODX logs at core/cache/logs/error.log to see if that has anything interesting.