I have a vanilla install of WP
Using Custom post type UI plugin I have added a CPT called 'news'. Created some 'news' items and then looked in:
Expecting to see an archive of all 'news' posts
Get a 404.
I can view an item fine: http://mysite.local/news/myitem
I have tried playing with various options for CPT but to no avail. One post said I needed to create a custom archive, but I'm pretty sure if none is specified then it will default to archive.php.