Could you check the data retention settings in System Manager?
Start > All Programs > Solarwinds Orion > System Manager
File > Orion Network Performance Monitor Settings
In the database tab, check the Statistics Summarization parameters.
If the Daily Statistics are set to something under 60 days or so, it explains why you cannot see the data from September.
If the Daily Statistics value is set properly then something might be wrong and I would advise you to open a support ticket.
Does the issue apply to all the charts? Which data would you need to retrieve from the database?
Note that you can use the Report Writer to retrieve most of the data you are seeing in the charts without the need to dive into SQL syntax.
Start > All Programs > Solarwinds > Alerting, Reporting, and Mapping > Report Writer