That was about a time when I couldn't even look at posts. I closed out the window, reopened it, and all I got was an interminable white screen (not even a "Page Not Found" or "Server Error" message. And the forum had been responding sluggishly, taking several minutes for a post attempt to take. Possibly the site was experiencing difficulties at that time.
The other possibility is that you simply ran out of posts. That should not happen without warning, since there is a "Posting Limit" line above the post that will tell you "You have made XX post(s) out of your limit of XX". If you used up all your posts for the day, it should tell you "You can currently post 0 more..."