Most deindexations can be easily avoided by following the best practices for building PBNs.
My blog doesn’t get indexed at all.
You should have at least 10-15 blog posts with images, videos and rich text and 3-5 standard pages (Privacy, About, Contact). If that isn’t enough, build a few backlinks to it, preferably from popular Web2.0s and social bookmarks. That should get it indexed in a few days. You can also use a backlinks indexing service like BacklinksIndexer or One Hour Indexing.
If nothing works, you might be looking at a penalty.
My blog got deindexed.
Check the blog’s Health Score, you will find the most common reasons for deindexation there. If there are no recent blog posts, only text posts with links or too many 404/301, then the blog had a high chance of deindexation.
Usually the best course of action after deindexation is to just scratch the blog and create a new one. PBN is a numbers game so when you lose one, just add another. Do make sure you’re following the best practices for building PBNs though (see top of this page).
You can also check directly with Google for the reason. Open a fresh browser (Clear Browser History) and add the website to a fresh Google Webmaster Tool account (never add multiple websites to one account!). It will show the exact penalty your blog got hit with. Most common for PBNs are “Pure Spam” and “Thin Content”. Check what they mean on Search Engine Watch blog post.