Forum Moderators: open
I've a theory. When you reach a determinate amount of indexed pages (let's say, 20000) Google examines your site, testing wheather if content is possible to be larger or not. In my case, the whole forum were indexed, and, of course, it should grow and grow and grow ... and, finally, Google considers your site is not "interesting" (many PR2-PR1 on the site, very similar Titles, etc), and drops you.
Two "droped" sites of mine had forums indexed... just for a few time. And then, the crash.
But is just a theory (the truth is just "there").
Regards
If there is no apparent reason for it (eg. you didn't break their rules), you'll get back in, but you'll have to wait for one or two spidering cycles for significant recovery.
Watch your site logs closely - if Googlebot is not crawling your site, then it is some other issue.