Are you sure you have both the old and new user on the server? Is it only windows authentication? Do you know the SA account/password? You could also look at this. I ran into a similar issue a few weeks back where the Windows SysAdmin moved the VM from one domain to another without telling me. Oops!

I was able to fix it and it is not all that difficult.

http://sqlaj.wordpress.com/2013/01/25/no-admin-account-on-server/