Set your website in Google Webmaster Tools to see if there is some errors. If your website has too much 404 errors, Google can freeze its indexation for a while.
Add a xml sitemap to help Google to find all your articles. Put a link on your website that will redirect bots and users to the sitemap, add the sitemap in your robots.txt file.
You can try to fetch as Google in Google Webmaster Tools in order to get Google Bot to recrawl. You can also see how Google is watching your pages. Maybe your design is not enough Google friendly. If you have setup plugins on a WordPress, compare the date when Google has ceased to index your pages and the date of installation of the plugins to know if one of your plugin could be the reason.
You can check the Google page speed to optimize your website with the Nginx module.
You can ask to Google to reconsider your site. With Google Tools Reconsideration.
Check the Google guidelines :
Maybe you have made your website too much for the search engines and therefore you have selected very specific keywords but the content of your pages is practically empty. By consequent, you may have been hit by a low-quality content penalty. If this is the case, you should rewrite your pages and add much more data with several paragraphs. Or even delete these pages. And later ask to Google to reanalyze your posts.
You could also have too much duplicate content, too much paid links of hidden links, or affiliate links.
Use one of the best SEO analyzer tools to get other possibilities of this kind of issue.
Maybe you are using an old domain name that was used to spam the net. Check the history of this domain; if this is the case, you should think about getting a new one.
The links on your pages have to be under a few thousands.
Check if your server has the If-Modified-Since (with Varvy SEO tool) header that informs Google if the content has been modified since the previous crawl.
You can add the search to the disallow list of robots.txt to prevent the search engines to crawl infinite spaces.
Don’t forget to index your site on other search engines, directories. You can also post intelligible posts on boards (with your link in your profile) or comment articles on other boards still with your link (not directly in the comment). It is important to reference your website with other services. As Google will crawl these sites too and will come back to yours.
You could add nofollow to your external links, otherwise Google could crawl these urls and then simply stop to analyze the rest of your texts.
If your site is just too young (less than 2-3 months), or has been put online again recently, after a long moment offline, it could be normal for Google to only index its information once per two or four weeks. It may wait that you generate more and better insights for the visitors.
Check the source code if you don’t have “noindex, nofollow” in your meta on every pages. With noindex, Google will simply ignore them. With nofollow, it won’t crawl the direct links present with the respective content.