My site blocked from Google Search?

If Google suspects your site of hosting dangerous or spammy downloads, engaging in practices that are bad or dangerous to the user, or of being hacked, you will see a warning either in Google Search results or in your browser (or both).

Google Search results might show labels such as "This site may harm your computer" or "This site may be hacked" next to your site.
Your browser might display an interstitial page when you try to open your page, either from a link in Google Search results or anywhere else.
If you are a verified site owner in Search Console, you should receive an email from Search Console warning that your site is suspected of being hacked or containing content that is harmful to visitors. You can also see warnings in the Security Issues report for your site.

Example warnings that you might see in your browser

Your website contains malware.
The term malware covers all sorts of malicious software designed to harm a computer or network. Kinds of malware include (but are not limited to) viruses, worms, spyware, and Trojan horses. Once a site or computer has been compromised, it can be used to host malicious content such as phishing sites (sites designed to trick users into parting with personal and credit card information). Some hackers may even take administrative control over a hacked site.
If your site has been infected, it is generally because some vulnerability has allowed a hacker to take control of your site. The hacker may change the content of the site (for example, to add spam), or add additional pages to the site, usually with the intent of phishing. Alternatively, they may inject malicious code (malware)—for example, scripts or iframes that pull content from another website that tries to attack any computer that views the page. Learn how Google defines malware and unwanted software.
This site may be hacked.
Hacked content is any content placed on your site without your permission as a result of vulnerabilities in your site’s security. In order to protect our users and to maintain the integrity of our search results, Google tries its best to keep hacked content out of our search results. Hacked content is often of poor quality, and may be designed to mislead users or infect their computer or device. We recommend that you keep your site secure, and clean up hacked content when you find it.

See What can I do? below to learn how to handle these problems.

Why did this happen?

Google checks the pages that it indexes for malicious scripts or downloads, content violations, policy violations, and many other quality and legal issues that can affect users. When Google detects content that should be blocked, it can take the following actions:

Hide search results silently
Label search results as dangerous or omitted for a specific purpose
Add pages to the Safe Browsing list of dangerous sites, which is used by most major browsers. These browsers typically warn users in some way before visiting affected pages.

The material on the page or site might violate our policies, whether or not you intended to.

You might have hired someone to manage your content or search performance, and this person engaged in practices that violate the Google Webmaster Quality guidelines: for example, link purchasing in order to increase search rankings).
Your site might have been hacked without your knowledge by professionals who engage in bad practices for money or other nefarious goals.

What material can be blocked or labeled?

Material that violates a Google policy, a law, or has been banned for some other reason can be labeled or removed.