Inspect the liner to determine where exactly it's come loose from the track. The liner is most likely attached to the edge of the pool with a track that the liner is locked into.
Check for wrinkles on the pool floor. If there are a lot of wrinkles, you may be able to push them toward the edge to give the liner more slack in order to reconnect it. If it still doesn't reach, you will need to empty the water from the pool.
Empty your pool in the morning on a hot, sunny day. A heated liner is easier to stretch.
Get inside of the empty pool and stand on the liner in the center of the pool. Push the liner toward the edges of the pool. This is going to take some strength, so you might want to get the help of some strong friends.
Fill up the pool with 3 inches of water while you are still in it. The heat from the sun will continue to help the liner stretch, while the water will hold in place the area of the liner that you have already stretched.
Keep stretching the liner until you can get it back to the top of the track. Push it all the way inside to lock the liner back in place on the edge. Fill up the rest of the pool when you are finished.