This is surely a disappointing news for bloggers especially when it takes so much effort to build a reader base.
Why did these "hundreds of subscribers" unsubscribe from your blog in one single day ? The first thought that strikes your mind is "Did I write something bad that offended readers and so they unsubscribed in a fit ?"
Well, it's none of your fault in most cases.
It is normal to see minor fluctuations in subscriber numbers reported by Feedburner but if you notice a large variation (in three figures or more), the problem lies somewhere with the feed readers and RSS aggregators such as MyYahoo, Bloglines, Rojo, etc.
These aggregators are either returning incorrect subscriber count to Feedburner or not reporting the count at all and hence Feedburner may not be accounting for those readers. A similar issue happened recently when Feedburner removed the count of Rojo subscribers.
Rojo numbers could soon be back but the other good news is that Feedburner will now include MyYahoo! numbers in your subscriber Chicklet.
Your blog's RSS subscriber count is bound to travel north as MyYahoo! is one of the very popular readers out there.
Previous Feedburner Hacks:
How Many RSS Subscribers Does a Blog Have
How to Change the Text of Feedburner Chicklets
Add Feedburner Signatures to GMail Messages