You just grab it and drag it back down (grab it by left-clicking and holding down the mouse button as you drag. Grab it in a place not occupied by an icon).
Assuming you are using Win7, you can also RIGHT click on the "globe" and pull down to properties. Hit the "taskbar" tab when that window opens up and it'll allow you to place the start bar at the bottom. Hit the "lock the taskbar" option if you want to avoid that from happening again.
BTW, works just about the same with WinXP, 2000 and Vista.
