Can anyone tell me why the Page Rank and Alexa Rank of my website has dropped down?

[Jake]

Google page rank is mainly based on backlinks from other sites, if you have lost links from sites that have vanishing, have been lowered in rank themselves, it's possible that a review of your sit has caused more backlinks to be discredited and labeled unnatural, adding more outgoing links tend to leak link juice, though links to authority sites add to your credibility, it's hard to guess from your 2 sentences.

Alexa rank is based on measured traffic, using data collected from a small sampling of users.

Only if you are selling a site are those rankings really all that important, search ranking for desirable keywords tends to more directly correlate to site success.