Are you able to run a copy of the site locally? If so, then it is likely not the codebase. If it doesn’t work, at least you know it is something in the site codebase and not likely an actual server related issue.
Was the hosting company able to look at logs. This being a server error there should be something that can give some information about what is happening.
I’m not sure of your hosting setup, but can you install a clean version of WP in a different folder and then connect to the same database. You’ll likely get errors on the front-end (if it even loads) but if you can access the admin, then maybe move those core WP files into the install that is broken.
There are some other thoughts I have too, but if you want to start here, that would be great information.
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