Most likely you have accidentally changed the sort order.

At the top of the column that shows the date of the post, you should see the words "Last Post" and it should have a small triangle pointing upward. If the triangle is not there, or is pointing the other direction, click on "Last Post" to select it as the sorting column, and/or to change the sort from oldest first to newest first (each time you click, it will change the sort order). Newest first has the triangle pointing up, oldest first has the triangle pointing down.