Look at the column headings - click on the date heading. It should put them either into newest to oldest or vice versa. Click it again to change the order.
You've probably accidentally clicked on a column header.
Click on 'Date' or 'Sent'(or whatever else is at the top of the relevant column). Clicking once should put the emails into date order, either with the most recent messages first or the oldest first. Clicking again reverses that order. (Clicking a third time takes you back to where you stated from).