I've had a client who was just uploading a photo, got booted out of the CMS. Then the site 404'd all pages, and wouldn't allow login at all.
I had to go into FTP and delete the cache for this to start working again. Happy to answer any questions to try and help debug this and get it fixed.
Running latest PHP, and confirmed with the host it meets Statamic requirements here: https://docs.statamic.com/installing#requirements