I've searched this forum and was surprised to not find this asked before - Is there any way to change SA's password (we have managed to lose it somehow)? And we do have physical access to our SQL Server of course...if that helps.
I know that it has been asked (and answered) a few dozen times in the last year, but right now the forums are a bit confused... Apparently the server had a "melt down" over the weekend, and it isn't fully recovered yet.
The short answer (assuming that your installation is at least vaguely standard) is to:
1. Log onto the server as a Windows Administrator.
2. Connect to SQL Server using Windows Authentication.
3. Change the password for the sa login.