Here is another post about this kind of problem (link). To fix it you will need FTP access to the web host so you can follow the instructions here:
http://www.nextgen-gallery.com/how-to-rollback-a-version/ which works if you have FTP access to the web host where your wordpress install is located.
If you are using WordPress.com then you might have to start from scratch, not sure. Hope this helps.
To roll back and download a previous version visit this link: https://wordpress.org/plugins/nextgen-gallery/download/
@spiranimcorp & @sharphead – Thanks for sharing the roll-back link …
@marmaladyrouge – Can you provide a link to your site with the specific URL of the gallery display that is not loading? We can start there to see if there is anything we can suggest.
– Cais.
