This can happen when one of the background tasks (which have icons in the task tray) puts up a notification popup that needs acknowledgement and you miss it. The bar stays up until you acknowledge the appropriate task, but the problem is that these popups can sometimes disappear even though they haven't been acknowledged, after some time has passed. What I do is right click on the task tray (not task bar) icons to get their context menus to pop up (I usually have some suspected ones in mind so I don't need to go through all of them) and then the task bar retracts. If you want to do the same (in case this happens again), it's easier if you set the task tray to show all icons instead of hiding some of them.