why does my picture keep going ?

I've got a Techno sonic LCD TV witch i bought from woollies about three years ago, It works fine for a few weeks but then the picture just goes off, the first time it happened i put it away till i could find some one to fix it but, it just sat there for about 2 months, then my son tried it and it worked, but now its gone off again, I've tried new scart leads turning it of at the plug but nothing