First, try the usual fixes listed at the bottom of the post (if you haven't already).
If those don't work, try this.
If you are using iCloud with Keynote, go to icloud.com on your computer and check to see if the file is there. Download a copy you computer as a backup, just in case.
Back on the iPad, turn off iCloud in Keynote, delete the document, then turn it back on again and wait for the document to sync again. If it does not, go back to iCloud and if the document is gone, upload it again from the computer.
Hopefully the iCloud copy was good and you're back in business. If it has the same problem the file was probably corrupt on both the iPad and iCloud.
In that case the only thing you can do is try to restore the iPad from your last backup, and hope the file was still good then. Be aware that a restore from backup turns back the clock for the entire iPad; so make copies of everything you've done since that backup. Better yet, always make off iPad copies of anything you consider important.
I don't consider iCloud.com a good backup, because syncing is automatic you can't be sure that changes or document deletions won't propagate to it before you can correct mistakes. A free Box.com account can be used to upload iWorks files via the WebDAV option. I consider it a great second copy location, though your files must be less than 100MB for the free account.
Good luck.
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When you have problems with an app it's a good idea to clear it from RAM.
Close/Clear the app: With the problem app closed, double tap on the Home button to get the multitask bar. Tap and hold on any icon in the bar until they jiggle, then tap the minus sign on the problem app. This will remove it from RAM and insure a clean start the next time you launch the app.
If that does not work try a restart,
Restart: Hold down the power button until the power off slider appears. Slide it off. When the iPad finishes shutting down, hold down the power button again until the Apple logo appears. Apple recommends you restart the iPad at least once a month.
and then a reset if that does not work.
Reset: Hold down the power and home buttons until the Apple logo appears. The iPad will reboot. This usually takes a bit longer than a restart.