We've reached a total number of posts that affects our performance when we load them all...especially on some forums like Blogging Central. Thus, we can only show threads that have had active posts in the last 6 months or year.
The older posts have not disappeared. You can still reach them.
To see older posts:
1.) Click on the desired forum.
2.) Scroll to the bottom.
3.) Under "Display Options", select the time frame you want. (Posts from all time is an option).
4.) Click the "Change" button.
You can also find older posts using the Advanced Search capability. You find this by clicking the "Advanced" button in the Search island to the right of every page.