Error 598 can stop SQL Server from starting right after a patch. In this case the script level upgrade failed because the service account could not create a temporary database file. Giving that account permission on the instance folder let the restart succeed.
SQL SERVER – A List of Various SQL Server RTM and Service Pack Number
Which SQL Server version are you running, and is it the latest? My list of RTM and Service Pack Number values to check your production server against.
Staying Current Without Chasing Every Update
Staying current on SQL Server takes a repeatable CU cadence, fast security response, and a patch policy your team can actually keep.
SQL SERVER – Order of Hotfix and Service Pack
What is the right order of hotfix and service pack installs? You do not need every older service pack. Install the latest service pack and hotfix and you are done.
Why SQL Server Has Cumulative Updates, Not Service Packs
Understand why SQL Server uses Cumulative Updates instead of service packs, what each CU contains, and how to plan a reliable patch cycle.