If you were able to see some reports on the Reports list, but they disappeared suddenly, it could be because some of your subscription packages expired. Each subscription package in Core includes a defined set of reports. If one of your subscriptions expires, the reports unique to it are no longer visible in the Reports screen. You can regain access to them by renewing the expired subscriptions. Check the Core Help for details. You may also want to check out this article that lists the reports included in each subscription package: Core reports by subscription packages.