In Windows, if your Start button and Taskbar seem to be missing, try moving the mouse to the very bottom edge of the screen. The Taskbar should pop up. If it does not, try moving the mouse to the top edge, the right edge, and the left edge. The Taskbar should appear on one of the edges.
To stop the Taskbar from disappearing, right-click a portion of the Taskbar without any buttons, or the clock portion of the Taskbar, and select Properties. It's likely that the Auto hide box is checked. Uncheck it.
In Windows XP and Vista, to stop the Taskbar from being moved to a different edge of the screen, right-click a portion of the Taskbar without any buttons, or the clock portion of the Taskbar, and select Lock the Taskbar.
If the Taskbar doesn't appear when you move the mouse to a screen edge, it could be that you accidentally made the Taskbar zero rows high. By default, the Taskbar is one row high, and you can increase and decrease the number of rows by dragging the top border of the Taskbar with the mouse. If the Taskbar is zero rows high, you should see a wide line at the bottom (or other) edge of the screen, and as you move the mouse cursor over this line, the mouse cursor will change. Click in the middle of the bottom edge of the screen, and drag upwards. If your Taskbar had been on a different screen edge, try dragging from that edge towards the middle of the screen.
You can use the Taskbar and Start Menu Properties window to change other Taskbar behaviors. For instance, by default, the Taskbar is always on top (it cannot be covered by other windows). By unchecking Always on top, you can allow other windows to cover up the Taskbar. You can also remove the Taskbar clock. To access this window, right-click a portion of the Taskbar without any buttons, and select Properties.