First check that you haven't inadvertently dragged it to 'zero height', at the bottom of your screen:
Place your mouse cursor at the very bottom of the screen. If the cursor changes to a double-headed arrow, it means that it's on top of the upper edge of the taskbar. Simply click and, holding the mouse key down, drag the top of the taskbar to where it belongs.
If you don't see the double-headed arrow, you'll need to follow the instructions here:
http://support.microsoft.com/kb/318027
Chris