Here are some tried and tested methods for recovering PPT files:

METHOD 1:

1. Open MS PowerPoint
2. Click the Office button
3. Now select the corrupt PowerPoint file in your system directory
4. Click on the Open button. A list will be scrolled down
5. From the list, click "Open and Repair" button

METHOD 2:

1. Launch Microsoft PowerPoint on your system.
2. Create a new presentation file and save it.
3. On the Home tab, click the drop-down arrow next to New Slide.Now at the bottom, click Reuse Slides….
4. On the right-hand side pane, a short Reuse Slides window will be launched. Click the drop-down button next to Browse, and then select Browse File….
5. Now locate the PowerPoint file whose slides you want to insert to a new file.Once you select the file, click Open.
6. You can see a list of all the existing slides in the selected PowerPoint file.At the bottom of the
7. Reuse Slides window, select the Keep source formatting check box.
8. Right click the first slide in the Reuse Slides pane, and select Insert All Slides.
9. Finally save this new presentation and republish it.

METHOD 3:

1. Click Windows Start button on the taskbar or press it on the keyboard. Now click Search.
2. In the All or part of the file name box, type *.TMP.
3. In the Look in list, click either Local Hard Drives or even My Computer. Now click Search.
4. After Windows finds your TMP files, click Date Modified button at the top of the Search Results pane. If you do not see the button, you might have to scroll to the right.
5. Look for a TMP file created around the time that you lost your PowerPoint file. Note the folder it is in.
6. Open Microsoft PowerPoint. Go to the File menu and click the Open button. 
7. Click the arrow next to the Files of Type box at the bottom of that dialog box, and then click All Files.
8. Navigate to the folder where the TMP file is located. Now click TMP file, and then click the Open button. 

OR, 

Use a professionally recomemded third party recovery tool like MS PowerPoint PPTx Recovery Tool​