I have lost all
passwords for a SQL Server instance, how do I give myself admin access again?
Run SSMS as Administrator (ie, with escalated privileges on the server). This should have sysadmin rights, and you can then find your login and assign rights as necessary.
Almost wrote "tights" there. You can probably assign them too, if you're running as an admin on the server.
Open Configuration Manager
Stop the server instance
Right, click >> properties >> advanced >> startup parameters
Add "-m;" right at the start of the string ensuring there are no spaces anywhere in the string
sqlcmd -S localhostfrom the command prompt
EXEC sp_addsrvrolemember 'domain\username', 'sysadmin'; GO
Ensure can connect from command prompt
sqlcmd -S localhost -E(if logged-in as that user to windows)
Remove "-m;" from startup parameters
Restart the database server