@BruBoy - If you delete the plugin, the data of all of your galleries will also be deleted (This is a WordPress function). What I would suggest is either refreshing the plugin files manually via FTP while the plugin is deactivated, and then reactivate the plugin once the process has finished. Or you might try deactivating all plugins (including NextGEN), switch temporarily to the WordPress default Twenty Twelve theme and then clear your browser cache. Refresh your site, reactivate NextGEN Gallery plugin only, and see if that helps. If it does, it may either be another plugin interfering with the lightbox display or it might be the theme you're using.
@Legende - Sorry to hear about this. Would you mind trying the same suggestion that I outlined for @BruBoy and let me know if that works for you as well? Deactivate all plugins > temporarily switch to WP Default Theme > clear browser cache and then reactivate NextGEN Gallery plugin. See if that helps the images display better.