Rejoined a game in the end (Survival) and dozens of Dragons spawned, making the game unbeatable

My sister and I were trying to kill the Ender Dragon in a survival world, and she had to go do something, so I left the game. She came back, and so we rejoined. Once we did, about a dozen Ender Dragons spawned. They killed my sister, and almost killed me before I left, and now we are left with an unbeatable and unplayable world. Help.