Today I saw my website's page rank gone down from 2 to 0. It happened once before as well. I don't remember spamming anywhere and I don't have too many posts on my forum. I heard they are very strict about forum websites and frequently try to keep the rank down. Have anyone of you experienced this?
Your PageRank can drop for a variety of reasons outside of your direct control. For instance, if your PR is mainly coming from site X, and site X suddenly stops linking to you or loses its PR for some reason, then you will experience a PR drop.
That said, most of the time it's probably going to be something you did. For instance, if you link to a spammy, low-quality site or engage in some kind of unethical practice that Google catches wind of (e.g. PayPerPost a few years back). Some SEO practices may net you a rank increase in the short-term, but once Google changes their algorithm, you could lose that PR.
That said, if you can't "remember" whether you spammed anywhere, then you've probably been spamming, which builds low quality links and can itself lead to Google penalizing you for engaging in spamming.
"I heard they are very strict about forum websites and tries to keep the rank down frequently. "
Do you have a source on this? I have found forum websites tend to rank poorly for a combination of reasons - forum software tends not to be very search engine friendly, and the quality of the content of the forums can be incredibly poor.
On the issue of pagerank, I personally don't worry about this too much, peaks and troughs happen for a variety of reasons, changes to the algorithm, back links get removed, sites get hacked etc. If you have good content and don't induldge in dodgy practises (spamming etc) your ranking should return.