You need to be more detailed about this, as you try different things.
1) If you select a photo in Aperture that is not corrupt, and press the M key, that will show the Master without any adjustments -- does that work? If you now do that with a corrupt image, what is displayed?
A Preview is what you see in the viewere in Aperture, A thumbnail is what you see in the Brower or strip view of many images.
If your Aperture Library is Managed it means that when you import, the settings are to store in the Aperture Library, as oppose to put them someplace else, or if already on the computer, Where they before import. If it is Managed, then in the Finder you can control-click on the Aperture Library, and choose Show Package Contents. Once in the Package, you can look for a folder named Masters. In that folder you will see a folder or each year, and in each year you will see folder for each month, and within each month you will see a folder for each day, and within a day there will be one or more folders, which if opened will show the Master images of a particular session of importing on that one day. Can you do this?
If your library is not Managed, then more info will be needed, but basically in that case you can click on File, and choose Show in Finder and it will provide you with info for a path to where the Masters are located.
Ernie