1. 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"
2. 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.
As one of additional resources, visit Microsoft community to know more about resolutions for corrupted OneNote files...
http://community.office365.com/en-us/f/172/t/155975.aspx
