Could of been a penalty yes, is it a forum? Any duplicate content?
Maybe a few quality backlinks dropped.
Best think to do is not so much worry about it and keep creating quality content and backlinks.
Stuff like this happens without any penalty. What happens is your competitors knock you down and their sites get indexed. Just the way the search engine works. Websites go up, websites go down.
But, I would still check if your site violates any of Google’s terms.