Try exporting from CS5 to idml then opening that and saving as an InDesign Document. If that doesn't work then I'm afraid the only non costly option will be to start again... But next time do that at the beginning, or better still export to inx from CS4 and open that in CS5.
I´ve got following problem. We design our annual school report in InDesign. In order to keep the required work at a minimum level, we use the document of the previous year, convert it into the currently used InDesign-version and replace the content where necessary. The general layout stays more or less the same.
This workflow is GUARANTEED to eventually end in disaster, and your day has finally come. At least you can still open the old version. "Recycling" old documents over and over by doing a Save As to a new name and deleting the old stuff leads to cumulative samll corruptions, and eventually the file fails under the weight of too many problems. ID is better than Quark, or especially Pagemaker in theis regard, but it still ends badly eventually.
What you want to do is make a new file. Open the old one and copy the stuff from it that stays the same from year to year and paste it into the new file. Save the new file as a TEMPLATE. Add any styles or swatches you need, and test to make sure you didn't bring in any fatal errors when you pasted from the other doc. Exporting to .idml, opeing that, and saving that version may be even better. For all new versions of the report, open the template, which will give you a fresh untitled copy of the template with everything you need and nothing to delete, and you'll be starting as clean as possible each year.