I had the same problem where I upgraded from MODX 2.8.x to MODX 3, and articles disappeared from he resources list. All resources were gone there initially until I upgraded Articles too, but even after upgrading articles, the articles container had gone and blogs vanished from the website.
I was able to fix it by upgrading Articles and then editing the class_key column in modx_site_content for any articles or article containers.
UPDATE `modx_site_content`
SET `class_key` = 'Articles\Model\ArticlesContainer'
WHERE `class_key` = 'Articles';
UPDATE `modx_site_content`
SET `class_key` = 'Articles\Model\Article'
WHERE `class_key` = 'Article';
So to fix:
- Update modx
- Update Articles
- change the context_key in values for article content
- refresh cache