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?
You can check if your page is still included in the index with a search like:
If it's there, then it could be duplicate content issue. If it's not there it might have been penalised. Difficult to say what exactly happened without any more details.
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 ?
If you have google analytics activated with your domain, you can check why it is not being indexed. Most common reasons being, too many flash adverts, spam like feel, too many reports about a particular page.