I'm assuming the ones that have come loose are the ones over the whell arch - The Bezels as they are called are attched to the bodywork by pop studs or metal screws with a plastic nut on them - you usually have to take the wheel arch splash trim off to get at them which themselves are fastened to the chassis by another different type of fastener - pesky job to do - The most commmon reason for trims to come loose/off is going too fast through puddles ( have lost several this way and still haven't learnt ).
Best go to Vauxhall and ask them for the correct fasteners before you start dismantling - not very expensive.