If you restore the database then you shouldn’t need to import anything from uSync. It could be that the uSync files are out of date - I would take the database as the source of truth, as that was running on the existing server.
Was there content in the site on the existing server? If so, then a backup and restore should be adequate. You may want to double check the existing agency have given you a backup of the correct database and that you didn’t create a new database by unintentionally, either when setting up the instance or running Umbraco for the first time.
I don’t know what your server control panel looks like, but if you have the facility to restore the database again I would try that first.
There are content in the existing server (it’s still live and hosted with the previous agency). I also have admin access to this umbraco site.
However, I’m not sure how to check if the database was the issue.
Based on what they previous agency said, everything is provided, including the database backup. However, the files he provided was source files, I updated some settings in appsettings.json, and used Claude to compile - I’m not sure if this process corrupted the files?
Would I be able to buy your time to get this fixed if possible?