Check if you have a backup of the file before it was corruped.
In OneNote, go to File–>Info–>Open Backups, and open the latest backup. If the file looks fine, you can drag it into your notebook and delete the corrupt file. Otherwise, KB article 825779 describes how to repair a OneNote file. I'd first create a back-up copy of the corrupt file in case something goes wrong and you need to try again. Then click Start–>Run, and type:
onenote.exe /forcerepair "filepath"where filepath (in quotes!) is the full path to the file that is corrupted. You can get this path by hovering the mouse over the section tab in OneNote. It's probably something like, "C:\Documents and Settings\your-name\My Documents\OneNote Notebooks\notebook-name\section-name.one" If you still have OneNote 2007 installed, be sure you run the 2010 version of onenote.exe (instead of typing onenote.exe above, type "C:\Program Files\Microsoft Office\Office14\onenote.exe")
In case you can't resolve the issue click here OneNote Repair Toolbox
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