During a recent consolidation exercise I was migrating one database at a time and then taking the old copy offline, unfortunately one of the DBs had (un-knowingly) been my default database and next time I tried to log in I received the dreaded "Cannot open user default database. Login Failed." Message.
In my case it was to execute the sp_defaultdb stored procedure using the OSQL utility, for example…
osql -Sservername -E -dmaster -Q"sp_defaultdb 'username','master'"
