ANSWER: Dear Prashant,
first of all thank you for your question.
To open your file .pps when PowerPoint can not do it, please do the followings:
- Open a new presentation.
- Go to Insert -> Slides From Files.

- Click on Browse and search for your .pps file.

- Click on the .pps file which you could not open, and then click OK.
- Click on Insert All, and then close the Slide Finder.

All the slides belonging to that .pps file should now be visible in this new presentation.