If your SQL admin has left and you do not have the credentials.... Microsoft has a great article to reestablish credentials.
There are a few things to note:
When performing the PowerShell steps in the article, be sure to explicitly run it as an admin.
When specifying the instance, if it was installed as a default SQL Server on your server, the following are likely what you need to know that the article isn't completely clear about specifying:
a. The "instance" will be the name of the sql server: "mysqlserver"
b. The default service name is "MSSQLSERVER"
The steps will require the SQL Server to be shutdown so all of your dependent applications will need to be stopped/quiesced
The link to Microsoft's excellent document is here: learn.microsoft.com/en-us/sql/database-engine/configure-windows/connect-to-sql-server-when-system-administrators-are-locked-out?view=sql-server-ver15&redirectedfrom=MSDN&viewFallbackFrom=sql-server-2014Â