In SEO Chat Forums I heard a lot of complaints about ranking problems. Below is the short diagnostic checklist that you can used to determine if your site is banned, penalized or just lose to competition. This test assumes your site is live for at least 3 months. As a background:

Banned: This is to signify that no pages of your site are included in the Google index.
Penalized:This means that your site has some issues in violation to Google guidelines or some non-conformance that will make your site appear spam to Google thus lowering the rankings.
Just lose to competition: Your site does not anymore appear relevant to Google because of some content issues or links, and your competitors are doing better than yours, which results to down rankings.

Step 1: Check if your site is indexed in Google. Method: Copy and paste this to Google search box:site:www.yoursite.com, if your site is using non-www version, site:yoursite.com

Diagnosis: If there are no results, your site is “banned” and check your site against Google quality guidelines. If there are results, proceed to Step 2.

Step 2: Type your domain name in Google, in the example above, the domain name is “yoursite”

Diagnosis: If you cannot find your site in the Google Top 20. You are most likely “penalized”. If you can find your site, proceed to step 3.

Step 3: In your indexed pages, select an indexed page with at least 5 words in the title. Copy and paste that entire phrase to Google search box. If you cannot find your page in Google Top 20, your site looks “penalized”. If you can find it, then you just “lose to competition”.

Recommendations: If the site appears banned or penalized, it needs an site investigation and submit Google reconsideration request to be safe after corrections. Otherwise if it just lose to competition, your site needs an SEO improvement or a campaign.