Rich has explained it as a caching problem, whereby your computer ‘helpfully’ remembers what the page should look like, and unhelpfully forgets to show the link to post a reply.
He suggested hitting the refresh button on your browser, which sometimes works for me, and sometimes doesn’t. I hit it numerous times, and then finally it does work.
If you really can’t get in, the go up to Tools and clear your cache. Then hit refresh.