Assuming that you are speaking of the reports referencing alerts then -
There are several kinds of Backups you can perform:
Event Tables (Alerts and Packetlogs)
The All-tables includes both the Config tables and the Event-Tables.
The Config Tables includes the configurations, policies, sensors, filters, acls etc. but no alert data.
The event-tables backup includes all the alerts and packetlogs only.
If you only restored the config-tables backup then the events were not restored so there is nothing to report on.
Your reports for non-alert-related data should still be working if this was the case though. (Example: The "Version Summary" or "Policy" reports should still work fine.