Your chat window is either maximized or moved to the right so that the minimize button is off screen. You might try clicking the maximize button to reduce the window to less than full screen. Horizontal and vertical scroll bars should appear at the side and bottom of the page. Use the horizontal scroll bar at the bottom to scroll as far right as the page will allow and see if you are able to see and click the minimize button of the chat window.
If this does not work you might try to temporarily set your screen resolution to a larger size. This should shrink the windows on your page by a relative amount. You might then be able to see the chat window minimize arrow at that point.
If all else fails, ask your instructor to temporarily take chat out of the navigation menu on the left of the page. This will eliminate the window completely and then it can be re-added and your chat window should be back to normal.
Hope Some of This Helps.