There is a break character where the overset text begins or
The text has an intended Keep setting or a Break style assigned.
Let's start with the first. Choose Type > Show Hidden Chararacters. Do you see an unexpected blue character where the text begins to be overset? Delete it. Or just press Backspace at the beginning of the overset text and then press Enter. Does the text reappear in Layout View?
Next compare the styles in Story Editor view since you can't see them in Layout view. Be sure the overset text doesn't have an unintended character style or paragraph style and that overrides are removed so that they match the text that is still visible.