SQL Server 2014 All Administrator Accounts Disabled
You need to start SQL Server in single-user mode.

The documentation Connect to SQL Server When System Administrators Are Locked Out says:

Start the instance of SQL Server in single-user mode by using either the -m or -f options. Any member of the computer's local Administrators group can then connect to the instance of SQL Server as a member of the sysadmin fixed server role.


See the page linked above for further considerations and step-by-step instructions.