Problem: without thinking about the ramifications, you've taken your default database in SQL Server offline. When you try to login through SQL Server Management Studio (or Query Analyzer) you get an error message because the app couldn't switch to the default database. Obviously, one feels pretty dumb after doing this. Thankfully it happened to me in development, not production!

Solution: It turns out the solution is just as easy as the mistake: when you log in, click the options button and choose a different database. More detail at public class:ben harrell.