I presume this is a beaded liner, concrete pool.
If so, pump some water from the pool to behind the liner (called floating the liner) this will let you easily move the liner into place again (you will know when there's enough water by how easily you can get the liner bead to meet the receiver track), use liner loc (a product available at the pool store that helps hold in older beads that continually want to pop out). Work your way towards a corner, leave a small portion on the corner open, pump the water back into the pool and fasten the last little portion in the corner.