If you have already converted the site and the listings are not available in the backend, the first question I would ask is if you have a backup of the old site. The easiest thing may be to export them from the old site and import into the new.
That said, we’d be happy to take a look at the issue. Please include the following credentials so we can look into the converted site.
Please make sure to include all these credentials. Database is needed because the posts are probably still in the DB.
WordPress Credentials
WP Admin Login URL:
WP Admin Username:
WP Admin Password:
FTP Credentials
FTP Host:
FTP Port:
FTP User:
FTP Password:
DB Credentials
DB PHPMyAdmin URL:
DB Name:
DB User:
DB Password:
Just in case, I recommend you proceed with restoring a backup to another site, and follow up here with access to that as well.