If, after successful upgrade to 11.3, the Web UI login fails with 'Database Error Not connected to PostgreSQL MASTER Server localhost 5432' ...
If Web UI Login fails with error: You do not have permission to log in. Database Error Not connected to PostgreSQL. MASTER Server: localhost 5432
If the "postgresql-x64-9.6" service is not running even after trying to start it then check Windows file system permissions because it runs as NETWORK SERVICE (NT Authority\Network service) and needs permissions to new folders introduced in the upgrade, eg, D:\PrognosisData and D:\PrognosisFiles. As well as file system permissions also look at adding exclusions into the security / anti virus software for these new base folders so that it can load DLLs. These are new in 11.3 and the rest of Prognosis still runs as LocalSystem and under the, eg, D:\Prognosis folder where it was installed. This service log file can be useful, eg, D:\PrognosisData\PostgresData\pg_log\postgresql-2017-12-12_000000.log
For a less common scenario, if "postgresql-x64-9.6" service is running fine, ensure that the PASSWORDS configuration in Prognosis has the new lines added by the upgrade PQLostgres and PQLrognosis as outlined in Online Help here Security > Network Layer > Postgres Database.
Note that this post uses an example of Prognosis installed on the D: drive but applies equally to installations on C: drive or any other drive letter. If you need further assistance make sure to open a case to IR Support.