It may be because of Google Panda, the most recent Penguin, or perhaps the fast Cheetah (ok this doesn’t exist yet). It doesn’t really matter what algorithm has penalised your web site.
If your traffic report shows a fall in visit, and your ranking positions are no longer the same, well it would be better for you contacting Houston, because clearly you have a problem, pretty much like the Apollo XI and the problem they have while landing. Yes, your users have the same problem, they can’t land although they are reaching your site and not the moon.
What should I do if my blog has been penalised?
In order to proceed with such an investigation, it is important to ascertain whether the loss of rank affects the site as a whole or just a limited number of pages.
Therefore the first step is querying search engines to ascertain if domain is still indexed. Visibility problems may sometimes affect those web site managed by multiple departments (or companies) due to the publication of pages or configuration files not yet finalized that end precluding the visibility or contribute to the de-indexation of the site.
Assuming the site is still indexed, a penalisation is in progress, which nature (manual or algorithmic) needs to be ascertained. The two major SEs Webmaster Tool consoles offer a great value of free but site-specific information revealed in accordance of the most recent algorithm updates.
Although the consoles don’t provide constantly up to date information, whenever a manual penalisation is on-going, Google is keen to send a message informing something is potentially wrong. If no messages are available in the console, everything leads to a site algorithmically penalised (loss of ranking) and reasons may span in different areas. 
So confusing, can I have a checklist?
You askes, and here it is. A checklist to start the investigation may look like:
- The most recent changes made on the site; a layout change made SEs bot to entirely re-crawl the site, hence the pages are no longer appearing as a result of temporary actions; it is always cautious to analyse the trend for a week or two;
- Whether or not the site is participating in unnatural link building programs or perhaps an heavy link building strategy that featured always the same anchor text; the most recent Penguin update is limiting the value of link resources whilst penalising artificial network regardless the time they have created (some sites have been affected for links generated over 6 years ago);
- Content on site is not original and useful for the users (e.g. spinned content); the most recent Panda update is now heavily penalising duplicate content;
- The web site is over optimized or uses techniques that deliberately violates the webmaster guidelines (e.g. keyword stuffing);
- The site has technical issues not properly addressed (e.g. canonicalization issues, trailing slash, etc.)
Another important aspect is the content copywriting strategy adopted. As per my experience, if your blog belongs to a big organization, the big risk is the blog has been thought as a tool to support the brand, hence limited SEO activities are conducted to support the blog optimization. If this is the case, in the light of the most recent Google algorithm changes, this shouldn’t come as a surprise at all. If a company centric content, with no external links (because probably some consultant suggested a link juice dispersion) and same anchor text over optimisation doesn’t sound new to you, that’s the cause of your drop.
The limited keywords selection (generic keywords used in the aim to boost the ranking performances) usage is now considered a spam technique, which has certainly contributed to narrow the ranking probabilities over time.
As market landscape changes constantly, the above factor if combined with the competition aspect, have certainly had the dramatic results noticed in the ranking report. So before blaming search engines because of your ranking results, start reconsidering what you have done and act accordingly.
As you can see, the above loss of ranking causes are pretty much the same I was talking about a year ago.
Update: as Google has released its own disavow tool, this should now become part of the weeding exercise and SEO management process.