could be something simple like you attempted to logon to your computer using a online windows account and your access to the internet was down at that time. if you are using a local logon your profile is in a relative path, if you add another drive and it has a copy of windows, then you might have two accounts with the same name but different SIDs (secruity identifiesrs)
By default, the profile resides in the following location:%SystemDrive%\Users\UserName
%systemdrive% is a enviroment variable that will point to drive that where your windows root is located.
that is, i f you have a copy of windows on drive c: but boot with a copy on drive d: but %systemdrive% is set to drive c
then windows will boot from drive D as a windows root, it will validate the username from the user database on on drive d
but when it goes to look up your user profile it will go to drive c: and find the profile there but the SID will not match the user SID and you
should be prevented from loading the profile.