It sounds like the PDF file you were using in the Books app on your iPad has become damaged and is now unable to be opened. Unfortunately, there is not much you can do to fix a damaged PDF file.
One thing you can try is to see if you have a backup of the file that you can use. If you have a backup of the file on your computer or on a cloud storage service like iCloud or Dropbox, you can try opening the backup copy to see if it is still intact.
If you do not have a backup of the file, you may need to try to recreate the file or find another copy of it. If you have written notes on the file, you may need to try to re-create the notes on a new copy of the file or on a different type of document.
I'm sorry that I cannot offer a more straightforward solution to your problem. I hope that this information is still helpful and that you are able to find a way to access the PDF file that you need. Good luck with your exam!