WordPress 3.6 and NextGen 1.9.13 (and above) Gallery Alert
Do NOT upgrade WordPress if you have a NextGen Gallery operating on your website.
There is a known bug that will cause your gallery images to become faded out and disable the navigation controls. Once you upgrade you can only roll back the database and upload older files.
The programmers at http://www.photocrati.com/ (Creators of the NextGen Gallery) are currently working on a fix for the bug but at this moment there is no solution.
I recommend visiting this page for more information in their forum:
If you need to rollback your WordPress install to fix this then here is the link page for archived WordPress downloads:
Here is the download page for archived NextGen versions:
We have experienced problems with this upgrade and are awaiting a solution. We will post updates when there is a fix.