HTTP ERROR 500 - afraid I messed up

Hi, I added my site booksinboxes.eu to my WPMU dashboard and ran a scan with Defender, who told me that there were 3 items that looked suspicious and were not needed for my site. I'm aware there was a warning that things could break down if I deleted the files but I still went ahead and did it without thinking of backing up first, and now the site is unavailable. Any idea what to do next?