Depends on what you mean by "deleted."
If you physically deleted the file, restore it from the Recycle Bin. Then
browse to the restored location and open the notebook again.
If you simply closed the notebook, browse to the old location on the disk
drive and open it. Try checking in C:\Users\John\Documents\OneNote Notebooks
(on my computer - your path should be similar).
Otherwise, a reinstall may be needed to get the file back.