This could be because the sleep mode has been selected / activated.
To deactivate the sleep mode press the Sleep Button on the remote control – and select off mode.
You have different sleep time options – 10 / 20 /30 / 60 / 90 / 120 / 180 / 240 minutes and the OFF function.
If there is no signal to the TV set for 15 minutes then the TV set will automatically switch off.