Every action in an Overleaf project—every keystroke, every upload, every deletion—is recorded in a continuous timeline. Therefore, recovering a deleted file is not about "undelete" but about "rewinding time." If you delete a file, do not panic. Do not close your browser. Do not delete the project. Instead, look to the top menu bar.
Because the History system tracks file-level changes, deleting a folder appears as a series of deletions (or a single batch deletion, depending on the Overleaf version). Use the surgical method above: browse to the historical version where the folder existed, open each file you need, copy its contents, and recreate the folder structure in your current project.
You have a project called "Dissertation." Yesterday, you accidentally deleted chapter3.tex . Today, you wrote 2,000 new words in chapter4.tex and updated bibliography.bib . You need chapter3.tex back without losing today's work.