How to access a fresh install of SQL Server 2012?
Since you have access to the machine itself, you can follow the instructions here to (re)add yourself as a sysadmin: https://docs.microsoft.com/en-us/sql/database-engine/configure-windows/connect-to-sql-server-when-system-administrators-are-locked-out

The short version of what that says is:


Stop SQL Server
Restart with the minimal configuration option -m
Log into the server with sqlcmd -S {your machine name or localhost}
Run the following commands:


CREATE LOGIN [Domain\LoginName] FROM WINDOWS;
EXEC sp_addsrvrolemember '[Domain\LoginName]', 'sysadmin';

Stop SQL Server
Restart it with the normal configuration