database – Can’t restore existing Drupal 9 site (no route found for “GET /”)

I have a site with Drupal 9.2.5 and the site was clobbered after installing WordPress in the web root.

I have a backup of the site database, and the sites/default/* files, but I just can’t get Drupal up and running again to restore it to the way it was.

I have tried installing again and placing the files back, but this doesn’t work.

I have looked at the Drupal docs and searched for migrating or restoring a Drupal site, but nothing seems to be working out.

Once I create a new database, and import all data from the old one, update my settings and restore the folders and files, I get this error:

The website encountered an unexpected error. Please try again later.

SymfonyComponentHttpKernelExceptionNotFoundHttpException: No route found for "GET /"
in SymfonyComponentHttpKernelEventListenerRouterListener->onKernelRequest()
(line 136 of vendor/symfony/http-kernel/EventListener/RouterListener.php).

I am not sure what to do here: the issue is obviously with routing but I am stuck.

I can get to the install script, but it tells me Drupal is already installed, and to empty the database and start again (not what I want!)

I have tried the upgrade script, but I get an error:

The website encountered an unexpected error. Please try again later.
SymfonyComponentDependencyInjectionExceptionServiceNotFoundException: You have requested a
non-existent service "access_check.db_update". in
SymfonyComponentDependencyInjectionContainerBuilder->getDefinition() (line 1032 of vendor/symfony/dependency-injection/ContainerBuilder.php).
SymfonyComponentDependencyInjectionContainerBuilder->getDefinition('access_check.db_update') (Line: 600)
SymfonyComponentDependencyInjectionContainerBuilder->doGet('access_check.db_update', 1) (Line: 558)
SymfonyComponentDependencyInjectionContainerBuilder->get('access_check.db_update') (Line: 186)
DrupalCoreUpdateUpdateKernel->handleAccess(Object, Object) (Line: 101)
DrupalCoreUpdateUpdateKernel->handleRaw(Object) (Line: 75)
DrupalCoreUpdateUpdateKernel->handle(Object) (Line: 27)

This is very frustrating. I can’t find any useful information on restoring an existing Drupal site and how to do this.