You probably closed the Table of Contents by accident. If you click on the "Windows" option on the main menu bar in ArcMap, then click on "Table of Contents" on the dropdown menu, your Table of Contents should reappear.
Hi Marie, grab the table of content title bar, and click and drag the TOC until your cursor hovers over the blue left pointing arrow, all the way on the left of the page in the middle. The arrow will highlight a darker blue. Release the mouse button and you TOC should be in the usual place.