Student or Learner
After writing a long subject, I pressed in "preview" and rules page poped up. and all what I wrote was gone, It's frustrating :(
I feel your pain. I'm bad about spending too long on posts, and I've gotten burned like that a time or two. Be aware that at many forums if there is no activity from your computer for a certain number of minutes (the number probably varies), your session times out and you are required to log in again. At many forums (however, not all) the text of a post you were working on appears to be lost. I'm not sure, but it might even depend on your browser.
I have found two ways of coping with this annoying phenomenon. First, if you have been busy writing a post for a while and want to preview it, select and copy your whole text before you hit "Preview." That way if you do have to log in again and your text is gone, you can simply paste it all right back in there. Make this a habit if you spend more than 10 minutes or so at a time working on a post without a preview.
Even if you failed to do that, however, all is not necessarily lost. Another way that has worked for me at a different site (can't guarantee it works here or on your browser) is to just hit BACK a couple of times. That may bring up from your browser's cache an image of the page where you were working on your post and then you can copy and paste.
Of course, simply being aware of the problem goes a long way toward preventing it. Just get into the habit of hitting "Preview" every few minutes.