If you have any children it is possible that one of them has pressed the timer on the remote control, this can be set from a small number of seconds to 1 hr or so before automatically switching off. Its usually a button that has a clock symbol on it, I'd suggest when you power on the tv that you press the clock button and see if it shows anything on the screen i.e a number, if so then keep pressing the button until you bring it back around to zero.
If you haven't children then I'm sorry I can't think of anything that be causing it except for a fault.
I presume you have plugged the TV from the power point as opposed to always leaving it on standby all the time, if you don't usually unplug it I would suggest trying that and leaving it out overnight and seeing if that may help.