how can some one hack into deployed public_html directory with laravel 5.4 deployed project

i've server ubuntu with laravel 5.4 and it works fine for some time, it has cpanel and deployed on siteground, after few days, i find weird files been uploaded to the public_html directory and server stop handling requests? here's a screenshot with files been uploaded, how can i identify the problem? that can't be a code problem right? i've no idea where to look? shall i change the host? i changed the password and everything i could.

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