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If you can unplug the power from the device, then do so. This will power it Off.
Leave it that way for a half hour, to cool down, and also to rest and reset.
Then try it again after that.
If you need other instruction, let me know. Also, if your C120 device is not responding even to holding down the power switch for a while, then it has indeed 'frozen up', and if the problem persists, it has a severe problem either with its power supply (wrong voltage level perhaps, or not enough regulation to reduce AC ripple voltage on the supply?...) or else with its circuitry. Let me know in Reply if you need further help, information, referral, or advice about the problem.
If this answers your question, please rate my answer. Thanks.