Nothing? Maybe you've renamed it accidentally. Click Start, type a word that's in the presentation but not in many other files, and see if that gets better results. If it pulls up a lot of results, click See more results so that you can sort the found files by date.
No luck? Try the Recycle Bin. Maybe you deleted the file.
Dead end? Don't panic. You can always restore the file from the backup you made yesterday.
You don't back up? I bet you will now. As for the file you desperately need to find today, you'll have to use file-recovery software. Before I discuss specific programs, I need to lay down one absolute rule about using them: Until you've either recovered the file or given up, do not write to your hard drive. Every time you do so, you lower the odds of successfully retrieving the lost file.
Following this rule requires you to use portable file-recovery software. Download the utility on another PC and save it to a flash drive. Plug that drive into your PC, and launch the program from there.
The rule also means that you shouldn't restore your file to its original location. Save it to the flash drive, as well.
With luck, either of the following two utilities will be able to find and recover your missing file. First, try the free Recuva Portable. It's fast and simple, it can preview image formats, and it works reliably most of the time.
If that doesn't work, try Software Shelf's File-Rescue Plus. It costs $40, but you can recover up to five files with the free demo version. Strictly speaking, File-Rescue Plus isn't portable, but you have a work-around. Install it onto another computer, and then copy the program file, FileRescuePlus.exe, to your flash drive. After you pay the $40, use Notepad to create a file called key.ini containing nothing but the license key that Software Shelf sent you after you bought the program. Place key.ini on the flash drive, in the same folder as the program file.
Follow these tips to escape from Windows 7 disasters, and you should enjoy some sweet dreams.