NaomiCooper,

If you did not save the presentation then, chances are it cannot be retrieved.

There is more powerful thing as microsoft powerpoint. This blog might be too effective - http://www.presentermedia.com/blog/2012/08/i-cant-open-a-powerpoint-presentation-in-2010/ 

You may read useful article: http://www.pptfaq.com/FAQ00108_Recovering_a_corrupt_presentation.htm 

Last one variant might be special solution for such desperate cases, talk you how to repair powerpoint file damaged - http://www.powerpoint.repair/