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What do you meant by locked ? Is it frozen ? If so, follow the steps below to unlock it.
Press Command, Option and Esc keys at the same time and this will bring Force Quit window. Select iTunes and click Force Quit. This will quit iTunes.
If that didn't work, then force turn off your Mac. Press and hold down the Power button of your Mac for 5-10 seconds and this will force turn off your Mac. Turn it back on. Check iTunes.
Please let me know if that didn't work, so we can continue troubleshooting.
Remember, this is not my final answer. If my solutions didn't work please don't rate me bad or poor. Instead please reply back to me and I will continue to help you until we fix the problem or you are satisfied.