This just happened to me too for the first time the other day.  I have Windows 7 with the Aero effects on and what I did to fix it is to hover the mouse pointer over the icon on the task bar (with Aero the little preview window opens) and then I right click within the preview box and click "Maximize" and it opens back up again.  Hope that made sense.  It's done it a few times and this fixes it each time.