Each time you save your presentation it replaces the prior version with the
current version unless you specifically employ a naming convertion and give each
save a unique file name.  For example, add an incrementing version number to the
file name with each File > Save As.

In your case, your prior version has been lost.

For the future, you may want to use "Sequential Save" which is a free add-in
that you can find at http://www.mvps.org/skp/seqsave.htm.  It will allow you to
always have a "before" and an "after" version of your file.