In SQL Server, if you have a default database set for a user and that database gets deleted, you may not be able to login (so completely stupid Microsoft). To change the default password, do the following.
From the RUN command or a command prompt
1) sqlcmd -E -S "MyComputerName\MyInstanceName" -d master
1>ALTER LOGIN [MyDomain\jdoe] WITH DEFAULT_DATABASE =master
2>Go