Login to SQL Server 2008 (R2) as Local Admin

Just because you are a local administrator does *not* mean that you have permissions to access/log into your SQL Server 2008 (R2).

However, you can regain access by starting the SQL Server instance in "single user mode".

All you need to do is to add -m; to the Startup Parameters box and restart SQL Server.

See also: How to: Configure Server Startup Options (SQL Server Configuration Manager) on Microsoft TechNet.