I restarted my computer while LockDown Browser was running. Windows Task Manager is now greyed out.
Posted by Respondus Support

If a computer is powered down while Respondus LockDown Browser is running, the Windows Task Manager will continue to be blocked when the computer is restarted. 

To restore the default settings on a Windows system:

1. Start the system from a power down (not suspension).
2. Start LockDown Browser
3. Immediately quit LockDown Browser
4. Log out of the Windows user account (do not use Windows Restart)
5. Log back into the Windows user account