my tv video dvd combi screen has altered how do i change it back?

I have a ferguson combi tv 20". The screen has changed so every thing is to big for example the clock on am tv is no longer visiable when going into menu half the options are out of view. It is wired to sky and the channels are only visable for one programme. When it is first switched on none of the menu fuctions are visable and it gradually improves but not completely. It has only recently done this and i have tried on my remote to alter it but there is no menu for picture size. I have also completely unplugged it for a number of ours in the hope it would reset itself.