I just wanted to add a high priority note/reminder to NOT uninstall or delete NextGEN Gallery 2.0.x from your WordPress admin area if you are having problems. If you formally uninstall or delete the plugin, it deletes all data, including your galleries.
That's standard for all plugins and all versions of NextGEN.
If you are having issues with 2.0.0, you can now update to 2.0.7 which resolves many issues. That update is available from your WordPress dashboard.
If you are having still having issues with 2.0.7 or later and want to roll back to the previous version (1.9.13), here are some step by step instructions for doing so safely: