In this instance the CA Performance Management r3.1 RTM release was upgraded to the r3.1 March Update Kit.
After upgrading the CA Performance Center, CA Data Aggregator and CA Data Collector servers the system appears to be down. The Data Aggregator service is running but showing as 'Unable to Contact' in the CA Performance Center web UI and the Data Collector services don't appear to be communicating with the Data Aggregator.
No data is available for reporting in CA Performance Center as a result.
Upgrading to a new release, or to a patch kit, can trigger the exhibition of these symptoms.
In the majority of situations where this symptoms are observed post upgrade, as long as the dadaemon and activemq services themselves continue to run, it is likely that nothing is wrong.
After an upgrade the DA needs time to complete post upgrade tasks before it gets to the often slow process of loading element configuration data from the Data Repository database.
Both of those things must take place before the Data Aggregator will allow the CA Performance Center server to communicate and synchronize with it.
All supported CA Performance Management releases
Allow time for the Data Aggregator to sync changes to the DR DB and gather element configuration data from the DR DB before it begins synchronization with CA Performance Center.
Use the 'tail -f' command against the (default /opt install path referenced) /opt/IMDataAggregator/apache*/data/log/karaf.log file to monitor DA start up progress.