I used PPT a week ago, but not there is a semi-transparent question mark over the icon in the dock. When I search, I can't find it on the computer,
Yeah, that's what it's telling you. Do you have the Office installation disc(s)? That's what you need to restore it. I'm not sure if you can install one piece of the suite or not. Backup first, then find the Remove Office application. Run that, then reinstall Office.