When the rest of the crew found Annabeth the floor of the room that the spider and Annabeth were in was becoming unstable. If the floor broke they would both be going to Tartarus. Just before Annabeth fell Percy caught her. They were hanging off the tips of his fingers. Jason and Piper were too involved in getting the Athena Parthenos to the ship that they didn't realize their two strongest leaders were falling into Tartarus.

They had brought Nico along. Hazel, Leo,Frank,and Nico were doing their best to help Percy drag Annabeth out but they couldn't. Nico and Percy promised each other to meet at the Doors Of Death. After that they fell.
After that grieve just fell on everyone. They started blaming themselves for what happened. Piper encouraged them to stop grieving and to set track for the mortal side of the Doors Of Death.