How to recover SQL Server ownership when NT account is not valid anymore

I have a local SQL Server Express 2005, for which I don't know the SA password. I always connected with my NT login and I used that when I created a DB I want to access now.

My company split and I am a member of a new domain now. There are no connections with the old domain anymore. My account resides in the new user domain and is a local administrator on the computer where SQL Server runs.

I can access the Server through the Management Studio, but not my user DB anymore. I am not recognized as a Server Admin anymore either. Obviously, local NT administrators are not automatically mapped to be SQL Admins.

Is there a way to do that at this stage? Can I somehow recover access to the SA level of access?

Thanks in advance.