Nextgen Gallery is awesome, it displays images in your WordPress installation in a way that is very customizable and using the plugin itself is a breeze.
Recently it got updated to version
1.3.4 3.1.5 1.4.1. If you're using a custom template, this upgrade most likely broke your installation. The reason seems to be that the upgrade removes or overwrites the view directory found under the wp-content/plugins/nextgen-gallery/ folder.
If you use the automatic upgrade feature from WordPress, then you'll need to put back your template from a backup. If you're doing the upgrade manually then just be careful not to overwrite your template and you should be fine. If you're not using custom templates, then this issue probably doesn't affect you.