Website which created using Drupal 8 Crashed - Restored from backup But not working

The website which was created in Drupal 8.4 version crashed when i was upgrading the Drupal. So i restored it back. But it is not working - The following message is showing: The website encountered an unexpected error. Please try again later. When contacted with the hosting company, i was told to consult with the Developer. But unfortunately he is not available for the service. The website contained some static pages and there also were option for the Teacher/Admin role person to Add pdf files under categories such as Test/Assignment/Notes etc. and the students who studies that corresponding course were allotted to download it. I am not having the screenshot of the website. However i am having a manual which i had prepared to help the client.