It may be that there is a temporary glitch, I`ve had it before where sites have fallen down the rankings unexpectedly, but after a couple of days to a week they were back at their previous positions.
It may be that you have triggered a penalty, as the others have said. If you log in to Google webmaster tools, there should be a message in there telling you what you have done wrong, and also giving a link to submit a reinclusion request.
One of my sites got a penalty for "hidden text", which wasn't really, it was just the title of the site under a graphic representation of the title, so people could see it if they couldn't view images. I got banned from google, but check webmaster tools every day, and got a message straight away. The site then vanished 2 days after the message telling me it was being banned. I then removed the text and submitted a reinclusion request. 2 days later my site was not only back in the index, but had risen to number 1
If you haven't check webmaster tools. If you have an account already set up you should have a message in their if you have triggered a penalty. If you haven't got an account, set one up asap (its very useful anyway), although i`m not too sure whether you get a message when you set up the account is your site is banned.
If you don't have an account at webmaster tools, you can set one up for free at:
www.google.com/webmasters/tools/
If you haven't had a penalty, it shouldn't be too long before your normal position is resumed, although each time this has happened to me i`ve moved up into a higher position than I had before.
I really don't believe that this is an excuse for Google to make money by forcing you into PPC. There are a lot of rumours about how google forces people to give them money, ups PPC rates at Christmas, blah, blah, blah, but from my experiences none of them are true.