help my 32av555d toshiba wont turn on?

my mother keeps turning the tv off at the plug when its not in use and she went to turn it on and no lights come up and it wont turn on any more
I have checked the components that are compatable with my other Toshiba tv which were the power lead and the ir board with the power button on it and they work fine.
when you plug it in you can hear the power supply making a high pitch noise for roughly 30 seconds then silence until you unplug it then you hear it again for 3 seconds
I have looked at the power supply board and cannot see anything obvious
where do I go from here
have a reasonable electrical comprehension and am handy with a soldering iron
is it the power pack or could it be the inverter or something else ???