|uhh, maybe explorer.exe crashed and you just need to reboot.|
maybe you've set it to "auto-hide," so it drops below the bottom frame of the monitor unless you drag your mouse cursor to the bottom of the screen.
maybe you've dragged it to a side of the monitor other than the bottom. In XP it is possible to drag and drop the start menu/taskbar to any side of the screen you wish, even the top.
those are the easy solutions. if you've already tried those things, it's gonna be something weirder.