Have you closed, and then re-opened PowerPoint?

When you saved the new one, did you give it a slightly different name from the old one? I hope so. I would save the new one with an added phrase such as Rev2. (revision 2)

Original = myslideshow.ppt
New = myslideshowrev2.ppt

If you saved it under a new filename, you might try opening PowerPoint a second time, and go to the File Open dialog box, and see if the old file name appears. If it does, double click it to open.

If you did not give it a new name, then I'm afraid you've overwritten the original, and lost it.

If you do find it, do a file, save as, and then follow the folders up the directory tree to discover what folder it is in etc.

Good Luck