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If it is frozen, then I'm sorry you have to force quit it. There is no other way to unfreeze it and no way to save the presentation now. It is too late But if the Auto recovery function is turned on, then Power Point will recover and open the presentation automatically next time you open it. Press Command, Option and Esc keys and this will bring Force Quit. Select Power Point, Force quit and open it. Does it open the presentation?
If not, then check the Autorecovery folder. In Finder, click on Go menu and select Home. Then open Documents folder. Open Microsoft folder and open Autorecovery folder. If you see any files there, then rename the files (to rename, select the file and press return key to bring the selection on name) and add .pptx at the end of the file name and then open it with Power point.
If it doesn't work, then I'm really sorry it is gone. There is no other way to get it back if it is not in the Autorecovery folder. Only in rare cases, Powerpoint wouldn't save it. If you are not satisfied with my answer, please don't rate me bad or poor. You don't have to rate me anything and you can close this page without rating if you are not satisfied.
If you have any questions or would like any additional information, please don't hesitate to ask.