About once a week, the site goes down and gives the following error:
Symfony\Component\HttpKernel\Exception\NotFoundHttpException: in Drupal\Core\PathProcessor\PathProcessorFront->processInbound() (line 43 of core\lib\Drupal\Core\PathProcessor\PathProcessorFront.php).
When I look in my root directory, a new folder is there called "vendor" and it seems to be full of different frameworks, including Symfony. I reinstall Drupal (version 7.54) and it works for about a week, then goes down again. I tried to to disable the modules to see if that was the issue, but when I try to access the modules page, I get a 500 error. This started happening on December, and I hadn't made any changes to the site at that time.