Restore a WordPress site

Hello!
The site, which worked for 3 years, stopped working. No changes were made to the CMS, theme or database files. Perhaps WordPress has been updated or some topics.

There is an open local copy - it opens only the main one and it takes a very long time to load. If you change the theme - everything works quickly.

The site on four pages is informational. There are no complicated plugins on it.

Im asking for help. You need to fix the site and make it open again :) All access is available.

In the response, please write experience with WP, approximate cost and terms.