Open the database administration dashboard from the Global Overview dashboard. Click on "Settings | Administration" and select "SQL Performance Investigator" and check if any errors appear there.
Next open the database dashboard "Activity | Session" which will show if the instance has workload. If the workload is very low, the monitored Oracle needs more activity to show data in SQL Performance dashboard. If the Oracle has very low workload based on the "Sessions" dashboard, SQL PI will not show data, it will show "No Dimension Activity" under Performance Tree for the Instance View.
The recommendation is to monitor the instance and increase the workload load to see if data appears under SQL Performance dashboard.
As a test run try this statement against the instance which might produce a bit more load on the instance:
from dba_extents a, dba_extents b, dba_objects;
If you done an agent upgrade and now the data is missing, please go to the database administration dashboard from the Global Overview dashboard. Click on "Settings | Administration" and select "Connection Details".
Here click on "Test connection" and if it fail please run the new grant privileges script as sysdba user. You can do that by clicking on the message "Insufficient privileges" which appears after the connection test finished. When clicking on the message the popup appears where you can enter the user credentials of the sysdba user. If you are not allowed to do a remote sysdba connection to the database, you can download the new grant privielges from the popup window.