If you have a lot of active plugins it can be quite a chore. But the best way to trouble shoot is:
1: Temporarily disable all plugins
2: Reactivate the plugin in question, in your case NextGen
3: Does it activate OK? Is it working OK?
- If not try getting a fresh copy or contact the plugin author (or his help forums if they exist)
- if all seems good:
4: Activate one other plugin
5: Test the questionable plugin again
6: Is it still working OK?
- if not, then the last plugin you activated may conflict with it, disable that plugin and go back to step 4
- if all still seems good, go back to step 4
This way you can determine which plugin, if any (it can also be a WP ver or theme issue, or something else) is causing a problem.
There's been times I've reactivated 3 at a time. It's a little faster and when things break I at least know it's one of the last 3 then I do those 1 by 1