Is this the first time you have tried to start SQL after moving the system databases or changing the service account? Can you log on using the service account and open the database files in Notepad? Have your Infrastructure people applied a GPO that could change folder permissions?
As SQLBill says, the problem could be the files are not accessible. If they are, then you need to look at permissions.
Original author: https://github.com/SQL-FineBuild/Common/wiki/ 1-click install and best practice configuration of SQL Server 2019, 2017 2016, 2014, 2012, 2008 R2, 2008 and 2005.
When I give food to the poor they call me a saint. When I ask why they are poor they call me a communist - Archbishop Hélder Câmara