Sudden inexplicable windows can’t load user profile error

Not much to say here, Microsoft’s KB has the fix, right here.

But just in case they change/move/break the KB the quick and dirty steps that worked for me are:

1. Click Start, type regedit in the Search box, and then press ENTER.
2. In Registry Editor, locate and then click the following registry subkey:
   HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList
3. In the navigation pane, locate the folder that begins with S-1-5 (SID key) followed by a long number.
4. Click each S-1-5 folder, locate the ProfileImagePath entry in the details pane, and then double-click to make sure that this is the user account profile that has the error.
   • If you have two folders starting with S-1-5 followed by some long numbers and one of them ended with .bak, you have to rename the .bak folder. To do this, follow these steps:
     • Right-click the folder without .bak, and then click Rename. Type .ba, and then press ENTER.
     • Right-click the folder that is named .bak, and then click Rename. Remove .bak at the end of the folder name, and then press ENTER.
     • Right-click the folder that is named .ba, and then click Rename. Change the .ba to .bak at the end of the folder name, and then press ENTER.
   • If you have only one folder starting with S-1-5 that is followed by long numbers and ends with .bak. Right-click the folder, and then click Rename. Remove .bak at the end of the folder name, and then press ENTER.
5. Click the folder without .bak in the details pane, double-click RefCount, type 0, and then click OK.
6. Click the folder without .bak, in the details pane, double-click State, type 0, and then click OK.
7. Close Registry Editor.
8. Restart the computer.
9. Log on again with your account.