Click, hold, drag!
I'm assuming you mean the start bar, customarily located at the bottom of the screen.
Right click it, make sure "lock the taskbar" is not checked. Simply hover the mouse over it, hold it down, and drag it down to the bottom.
To ensure this doesn't happen again, once again select "lock the taskbar"
For more useful infomation From Ms:http://support.microsoft.com/kb/962963