If you are a local admin on the box, you can follow this [step by step process](
http://timradney.com/2013/11/21/how-to-reset-the-sa-password-in-sql-server/) that I have used dozens of times to reset the SA password. In my cases it would be when a server has changed domains and our SQL accounts were tied to the old domain which can no longer be authenticated. This process shows you how to start SQL in single user mode, create a new account, make it SysAdmin, then you can connect using that account and correct what ever problems you have, be it resetting SA or what ever you need. Hope this helps you out.