SQL Server 2014 - Disabled All Admin Users
Yes, you can get back in.  :)


Go into SQL Server Configuration Manager
Select SQL Server Services
On the right side of the pane, find your instance and right-click for Properties
On the Startup Parameters tab, in the Specify a startup parameter box, type -m and then click Add. (That's a dash then lower case letter m.)
Click OK and restart.
MSDN suggests launching Management Studio as an administrator for 2014.  The traditional way is to use SQLCMD -E.  Either way.


(If it doesn't let you because that account is explicitly denied, close the window, run the command line as a different user, and try again.)

Once you're connected, you should be able to create a new login

CREATE LOGIN [YOURDOMAIN\SomeUser] FROM WINDOWS;
ALTER SERVER ROLE sysadmin ADD MEMBER [YOURDOMAIN\SomeUser];


or

CREATE LOGIN TempLogin WITH PASSWORD = 'SomeReallyGoodPassword';
ALTER SERVER ROLE sysadmin ADD MEMBER TempLogin;  


and/or enable the disabled accounts.

ALTER LOGIN yourLogin ENABLE  
ALTER LOGIN AnotherLogin ENABLE  


Because this is production and this requires a restart, you'll have to do it after hours.  Good luck!