Side panel of UPVC window has come away?

The top half of a side panel of my UPVC window has come away from the interior wall, perhaps due to strain from netting curtain which has been screwed into the panel (though this shouldn't happen). The wall behind seems to have crumbled slightly and dropped down and is causing the rest of the side panel to come away. Not sure how to tackle the 'bowing' of the side wall and UPVC panel? Can it be re-stuck on top of the crumbling wall? Do I need a new panel? Do I need a builder to straighten the wall up before a new panel can be installed? A previous owner installed these panels himself and I fear he was an amateur DIY- enthusiast. Please help.