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It all depends how far it has moved. If the movement has been small and mostly away from the walls, rather than shifting horizontally along the walls/floor, you can just raise the water level in the pool and things will go back to normal. For somewhat larger amounts of movement you can slowly raise the water level while brushing/tugging/pushing the liner back into position. If the liner has shifted too much you need to drain the pool and reset the liner (if it isn't too old) or replace it (if it is older).