What e-mail program are you using? In Thunderbird, if the dividing line has become pushed to the left, the left side of the Window border looks thicker where the boxes should be. If you are careful and accurate when clicking the mouse button, it should be possible to grab the dividing line and move it, rather than resizing the whole Window. Try approaching the line slowly from inside the Window until the double-headed arrow appears. Then click, hold and drag.
If it's Outlook Express, go to: 'View > Layout' and make sure the 'Folder List' box is ticked. Click 'Apply'.