Have you had instances when your DBA left you with a DB that no one has SA access rights to?
For any instance of Microsoft SQL install, including SQL Express, there is a mode which would enable a local admin on that SQL server machine to gain/assign access to DB.
There are few steps which you have to follow:
1. Stop all SQL services
2. Start a command prompt with Admin permissions
3. Navigate to:
C:\Program Files\Microsoft SQL Server\MSSQL14.SQLEXPRESS\MSSQL\Binn
sqlservr.exe -m “Microsoft SQL Server Management Studio” -s SQLEXPRESS or any SQL instance name
5. Start SQL Server Management Studio as local admin and assign relevant groups/users access to DB