My old box was changed to different domain and I could not use Windows authetication to log into the SQL server there as “sysadmin”. I forgot the SA user password as well. After some search I found the following solutions
Option 1 :
If there is any other SQL Server Login that is a member of sysadmin role, you can log in using that account and reset the password of SQL Server.
Option 2 :
If there is any other Windows Login that is a member of Windows Admin Group, log in using that account. Start SQL Server in Single User Mode as described here : SQL SERVER – Start SQL Server Instance in Single User Mode.
Create a new login and give it sysadmin permission.
Note : If you have SQL Server Agent enabled, it starts before SQL Server service. If you have enabled SQL Server in a single user mode, it will connect it first, so it is recommended to turn that off before attempting any of the above options.
I used the second option and was able to reset SA’s password.
Start SQL server in “single user” mode.
To start SQL Server in single user mode is very simple procedure as displayed below.
Go to SQL Server Configuration Manager and click on SQL Server 2005 Services. Click on desired SQL Server instance and right click go to properties. On the Advance table enter param ‘-m;‘ before existing params in Startup Parameters box.
Make sure that you entered semi-comma after -m. Once that is completed, restart SQL Server services to take this in effect. Once this is done, now you will be only able to connect SQL Server using sqlcmd.
Make sure to remove newly added params after required work is completed to restart it in multi user mode.
Now I was able to login to the SQL server with windows authentication.
I change the password for user SA.
Thanks to links from: