Cannot access any database in SQL Server 2008 R2 Express after moving workstation from domain to workgroup
I have been in this situation before. If you do not have the sa password and you are not a member of the sysadmin group then you only have one option remaining to you which is to start SQL Server in maintenance mode. This will allow you to reset the sa password and to add yourself to the sysadmin server role.

Here is a link that I found useful when I had to do this as it describes setp-by-step what you need to do. Good luck!

http://blogs.msdn.com/b/raulga/archive/2007/07/12/disaster-recovery-what-to-do-when-the-sa-account-password-is-lost-in-sql-server-2005.aspx