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Hello and sorry you're having problems. Offhand- NO, there is no way you could have locked yourself out, and the initial symptoms (running for 3 hours, or 30 minutes then shutting off), indicate a hardware issue within the TV.
Can you please tell me exactly what the power light on the front of the set is doing when you press the power button to turn the set on? Is it lit steadily, blinking, etc.?
Also what specifically "lights up inside"- the back of the TV, or the power button itself?
Are you able to see my reply?