When  all windows user accounts with sysadmin role  or sa account are disabled in SQL server then how to access the instance. Below are the steps to solve this problem
1.	Stop existing instance of SQL Server on which all windows user accounts or sa account are disabled
2.	Start SQL Server instance in single user mode with following steps
2.1	Open command prompt in admin mode
2.2	Run command ( I am running SQL Server 2012 instance)
C:\Program Files\Microsoft SQL Server\MSSQL11.SQL2012\MSSQL\Binn>sqlservr.exe -s
SQL2012 –m
3.	Open another command prompt in admin mode
4.	Connect to the SQL Server instance using command
C:>sqlcmd –S(local)\instancename –E
5.	Create login 
C:> CREATE LOGIN [loginname] FROM WINDOWS;
C:>go
6.	Add login to sysadmin role
C:> ALTER SERVER ROLE sysadmin ADD MEMBER [loginname]
7.	Now disconnect from SQL Server instance
8.	Stop SQL Server instance by pressing cntl+C and ‘Y’
9.	Start SQL Server instance in muti-user mode and now you will be able to connect using newly created login