This probably means that the backlinks you had were not good quality links and may have been deleted or ignored by google. Are you using the same method to check your backlinks as you did before? There is a huge difference for example between checking your links with google or checking your links with yahoo.
I never build Backlinks to a page on My blogs, but when I checked it has about 900 backlinks without Anchor text.
You have to be prepared for such things, but it's not a problem. I had same situation with my site, don't know why, but my backlinks numbers sumtimes are bouncing (i check it each week), I also had big decrease but it eventually turn out to be a temporary.
Google is making updations in its system. So, whenever Google update itself it does not consider some of your less valuable links(your website) increase links through blog comments and other sources. Check back links with backlinkwatch.com
In Feb this year I also see my back link suddenly change. It seem that Google use not only 1 database center to store backlink, so when they change database there will be a sudden change