And thank you for your question.
Well it usually takes quite a bit of pulling and pushing in order to get the liner locked back into the track. You shouldn't have to drain the entire pool, however you may need to lower the water level a foot or so for better and easier results.
Some methods of stretching the liner include the use of boiling water or a heat gun. Just be careful as to not drop the heat gun into the pool or burn the vinyl if you choose this route. You may be able to stretch the liner enough without the use of either of these methods. It will take some elbow grease however and may require an assistant. Just be sure you don't over stretch the liner or tear it. If it becomes apparent that you cannot fully stretch the liner back into the track, you may need to lower the water level some more in order to do so.
Once you get your liner back into position and into the track, you may want to consider purchasing a liner lock from your local pool distributor. This will help to alleviate any future occurrences of this problem.
I hope this answer has helped, and thanks again for your question.