The best way is to simply reset the password by connecting with a domain/local admin (so you may need help from your system administrators), but this only works if SQL Server was set up to allow local admins (these are now left off the default admin group during setup).
If you can't use this or other existing methods to recover / reset the SA password, some of which are explained here:
- Disaster Recovery: What to do when the SA account password is lost in SQL Server 2005
- Is there a way I can retrieve sa password in sql server 2005
- How to recover SA password on Microsoft SQL Server 2008 R2
Then you could always backup your important databases, uninstall SQL Server, and install a fresh instance.
You can also search for less scrupulous ways to do it (e.g. there are password crackers that I am not enthusiastic about sharing).
As an aside, the login properties for sa would never say Windows Authentication. This is by design as this is a SQL Authentication account. This does not mean that Windows Authentication is disabled at the instance level (in fact it is not possible to do so), it just doesn't apply for a SQL auth account.
I wrote a tip on using PSExec to connect to an instance using the NT AUTHORITY\SYSTEM account (which works < SQL Server 2012), and a follow-up that shows how to hack the SqlWriter service (which can work on more modern versions):
- Recover access to a SQL Server instance
- More on Recovering Access to a SQL Server Instance
And some other resources:
