PHP Fatal error: Class 'Drupal\Core\DrupalKernel' not found
Try the following:


Check if autoload.php file exists and it includes the right path to your vendor/ folder. 
Clear any external cache services if in use (e.g. Memcached or Redis).
If you're using PHP 5.x, you should use PHP 7.x instead.




To rebuild your site from scratch based on your composer.json file, try the following steps:


Backup and delete the /core and /vendor directories completely.
Delete the composer.lock file.
Run: composer install.
Run: composer update.


Source: D8 conflict resolution.