What Might Cause A Page Rank Drop?

[microlinx]

I have a site that has had a PR of 5 for several years.
Now, suddenly without any substantial changes to the site, the PR dropped to 4 last night.

Yahoo, MSN and other sites show thousands of sites that link back to mine, but Google only shows 56. Are there any ways (tools) to analyze this?

Did I miss something? Did they do another update or about to?

[SEOPTI]

I believe you missed a lot, just sit down and read webmasterworld for the next few weeks.