Windows 7 and 8 may occassionally corrupt a user profile, preventing someone logging onto the PC.  Follow the instructions in the following Microsoft article to resolve

https://support.microsoft.com/en-us/kb/947215

Log onto the PC using a different account (e.g. administrator) or in safe mode

• Click Start, type regedit in the Search box, and then press ENTER.
• In Registry Editor, locate and then click the following registry subkey: 
  HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList
• In the navigation pane, locate the folder that begins with S-1-5 (SID key) followed by a long number.
• 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.
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  • 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: 
    1. Right-click the folder without .bak, and then click Rename. Type .ba, and then press ENTER.
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    2. 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.
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    3. 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.
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  • 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.
• Click the folder without .bak in the details pane, double-click RefCount, type 0, and then click OK.
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• Click the folder without .bak, in the details pane, double-click State, type 0, and then click OK.
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• Close Registry Editor.
• Restart the computer.
• Log on again with your account.