I rebooted while LockDown Browser was running. Now the Windows Task Manager is greyed out.

If a computer is powered down while 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