Administrator Unable to Unlock a "Locked" Computer

This behavior can occur for either of the following reasons:  When the default screen saver is set to use a non-existent screen saver program. And/or When you use a corrupted screen saver that is password protected.  More Information.

Information About Unlocking a Workstation

The following registry setting is received every time the computer is locked:  Start/Run/Regedit

HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\Winlogon

ForceUnlockLogon
REG_DWORD

0 - Do not force authentication inline (default)
1 - Require online authentication to unlock

The preceding value controls whether a full logon is performed during the unlock process. This can force a validation at the domain controller for the user attempting the unlock process.

NOTE: If the value is not present, it functions as if it had been set to 0 (zero).

Lock Your Computer and Use Other Windows Logo Shortcut Keys

To use the shortcut, press the Windows logo key+L. The following list has different computer lock-up scenarios that are available to you, as well as other ways to lock the computer: Click Here.

Create a Shortcut to Lock Computer

Right click a blank space on the desktop, select new, shortcut. Copy and Paste this line: "rundll32.exe user32.dll,LockWorkStation" in the program location box. Click next and create a name for your shortcut, click finish.

Lock the Taskbar

This restriction is used to force the locking of the taskbar and restrict users from making any changes to its position. Start/Run/Regedit:  Navigate to this key and create a new DWORD value, or modify the existing value, called 'LockTaskbar' and edit the value according to the settings below.  Exit your registry, you may need to restart or log out of Windows for the change to take effect.
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer]
Value Name: LockTaskbar
Data Type: REG_DWORD (DWORD Value)
Value Data: (0 = Unlocked, 1 = Locked)