[Solved] Unable to start SQL Server Service after applying service pack Error: Script level upgrade for database 'master' failed because upgrade step 'ISServer_upgrade.sql' encountered error 5069, state 1, severity 16

Problem:
SQL Server service not starting after service pack / cumulative update installation.
Error Message:
Script level upgrade for database 'master' failed because upgrade step 'ISServer_upgrade.sql' encountered error 5069, state 1, severity 16. This is a serious error condition which might interfere with regular operation and the database will be taken offline. If the error happened during upgrade of the 'master' database, it will prevent the entire SQL Server instance from starting. Examine the previous error log entries for errors, take the appropriate corrective actions and restart the database so that the script upgrade steps run to completion.
Root Cause:
From primary analysis, the code ISServer_upgrade.sql is part of SQL Server DLL and it is no where located on the operating system.
Upgrading this script failing for some reason, which can be identified by starting sql server with trace 3601 and check the error log to identify the exact code running on SQL Server while updating the server.
NET START MSSQLServer /T902 /T3601
However, this DLL is part of the SQL Server, this will be fixed by updating the next service pack or cumulative update.
Troubleshooting Steps:
Start the SQL Server Service with trace 902.
           NET START MSSQLServer /T902 




Download the latest cumulative update on top of service pack.

For example, after installing Service Pack 2 on SQL Server 2016 RTM, if the SQL Server Service not started, install the latest cumulative update CU2 on top of Service Pack 2 while running SQL on T902.
After successfully updating SQL Server with CU2 stop the 902 trace and restart the SQL Server Service from SQL Server Configuration Manager.
NET STOP MSSQLServer
NET START MSSQLServer
Reference:
SQL Server build version: