It looks to me like your presentation is corrupted. If you still have a copy of the version of OpenOffice.org that you used, you can try to open it in there. If it opens fine, export it to ppt format (Microsoft Office 2003).
Do not try to save it from within Writer. If Impress will not open the file, then for sure your file has become corrupted.
To be sure, you can rename the file so that the file extension is .zip, and open it with Archive Manager. This will tell you if the file is corrupted (at the level of the compressed file) or if it is corruption in the xml files.