I have a website which had a particular page url that was indexed in google search results - in the first 10 results. Suddenly it disappeared. Now that page is not even in the first 100 results. What would be the reason? I am feeling that the page has be spammed by our competitors. Is it possible to avoid that, or can I find if that page has been spammed or not? Is it possible to find the particular page in a website is spam or malicious?
If you don't have a google-webmaster account - you should open one and you might find some clues in regards.
In case you were using blackhat SEO Technics - it might led google to remove your website from the search results.
In general, think about it like this: google tries to get the top results to point to websites that it considers to have top-quality and unique content. Is there anything you've done lately that might led google into thinking that your content is not unique ?