She reverts to being a private citizen. In the most recent presidential assassination, the Kennedy killing, there was a bit of a kerfuffle over Johnson's access to the White House. I wrote about this previously on the subreddit. Basically, Johnson was anxious to move into the Oval Office to project the image of continuity of government, but he was also wary of being seen as evicting the popular First Lady and her family, and the situation was further complicated by the fact that Kennedy's brother, Robert Kennedy, was now a member of Johnson's cabinet and the two men despised one another.

In any case, Johnson worked out of his office in the OEOB for three days after the assassination, and moved into the Oval Office afterward, but did not move his family into the White House residence until a few weeks after the assassination. Robert Caro's The Passage of Power gives an account of this:

McGeorge Bundy had told Johnson Friday evening that he would be able to move into the Oval Office Saturday morning, but subsequently the national security advisor had learned that that would not be a good idea, and, going to the Executive Office Building early Saturday morning—8:05 a.m.—he left a note for Johnson there, telling Mildred Stegall to give it to him as soon as he came in. “When you and I talked last night about when the President’s office in the West Wing would be ready, I thought possibly it would be immediately,” the note said. “However, I find they are working on President Kennedy’s papers and his personal belongings and my suggestion would be that—if you could work here in the EOB today and tomorrow, everything will be ready and clear by Monday morning.” Johnson, unfortunately, didn’t get the message. Emerging from his limousine at about 8:55, he didn’t go to the EOB, but walked into the West Wing instead—to the Oval Office—and walked in on Evelyn Lincoln as she was beginning to pack up Jack Kennedy’s belongings. “I have an appointment at 9:30,” he said. “Can I have my girls in your office by 9:30?”

That would give her a half hour to pack. “I don’t know, Mr. President,” she said. “Grief-stricken and appalled,” in a friend’s words, she walked out of the office and began to cry—just as Bobby walked in. Sobbing, she said, “Do you know he asked me to be out by 9:30?” .....

He would stay in his own home, too. Asked by reporters when the Johnsons would be moving into the executive mansion at the White House, Lady Bird replied, “I would to God I could serve Mrs. Kennedy’s comfort. I can at least serve her convenience.” Jacqueline and her children would move out on December 6th, the Johnsons would move in the following day. And although the address of The Elms was being printed in newspapers and television was showing pictures of the house during those three days, the number of persons standing outside remained surprisingly small. The President of the United States was living there—and, during those three days, the world didn’t seem particularly interested.