Ran the Craft Updater from Control Panel the current install is no longer working
Try running
    composer update
Via the command line while in ssh

Or download the latest Craft files and replace your files. Mainly vendor/craftcms it looks like.

Edit:

Update composer.json to this and try running composer update again.

{
  "require": {
    "craftcms/cms": "^3.0.35",
    "vlucas/phpdotenv": "^2.4.0",
    "craftcms/redactor": "^2.1",
    "craftcms/mailgun": "^1.3",
    "craftcms/contact-form": "^2.2",
    "indigoviking/liquid-letters": "^1.2",
    "lukeyouell/craft-contactformtemplates": "1.0.1",
    "doublesecretagency/craft-bootstrap": "4.1.1.2",
    "fatfish/navigation": "1.0.7",
    "superbig/craft3-templateselect": "^2.0",
    "craftcms/ckeditor": "1.0.0-beta.2",
    "verbb/navigation": "^1.0",
    "enupal/slider": "1.2.2",
    "studioespresso/craft-dumper": "^1.3",
    "topshelfcraft/wordsmith": "^3.0"
  },
  "autoload": {
    "psr-4": {
      "modules\\": "modules/"
    }
  },
  "config": {
    "optimize-autoloader": true,
    "platform": {
      "php": "7.0"
    }
  },
  "scripts": {
    "post-root-package-install": [
      "@php -r \"file_exists('.env') || copy('.env.example', '.env');\""
    ]
  }
}