You may have to start over from scratch with a new library as follows:
Starting over from scratch with new library
Start over with a new library and import the Originals (iPhoto 09 and earlier) or the Masters (iPhoto 11) folder from your original library as follows:
1. Open the library package like this.
2. Launch iPhoto with the Option key held down and, when asked, select the option to create a new library.
3. Drag the Originals (iPhoto 09 and earlier) or the Masters (iPhoto 11) folder from the open iPhoto Library package into the open iPhoto window.
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This will create a new library with the same Events (but not necessarily the same Event names) as the original library but will not keep the metadata, albums, books slideshows and other projects.
Note: your current library will be left untouched for further attempts at a fix if so desired.
If you still can't import those same files into the new library by this method they may have been damabed somehow. If you can find one you could try dropping a copy of it on this Automator workflow application, Convert to JPG and Embed sRG Profile which can be downloaded from Toad's Cellar and see if the converted file can be imported. If so you'll have to find and convert all of those files before adding the to a new library.
OT