That is quite strange. Have you tried deleting the plugin folder and reinstalling? Perhaps there is a file permission problem. What version of the plugin are you running?
I have never seen this issue before. Something has likely gone very wrong in your update process, something quite unique. If all else fails it is likely that backing up your site, and then reinstalling everything from fresh zips will fix it. It's a bit of a last resort, but if something happened during the update it might be necessary.
Let us know what happens if you don't mind. Thanks!
- Brook