It should pop right back in place. You'll likely have too massage it from both ends towards the middle. The friction from your fingers will help it stretch a little as you rub and rub and rub. Lol. It only takes a few minutes.
Then liner lock the whole pool. 120ft is about $30 on Amazon and it will stop it popping out there, or anywhere else.
Check the bottom of this link for a visual
www.troublefreepool.com