First, right click on the taskbar and ensure that 'Lock the Taskbar' is not ticked.
Then hover the mouse pointer over the top edge of the taskbar until you see a vertical two-way arrow. Hold down left mouse button and slowly drag the bar downwards until it becomes normal height.
If you 'overrun' and the bar disappears, hover the mouse at the bottom edge of the screen until the two arrow appears, and drag it back up.
Finally right click on the bar again and tick 'Lock the Taskbar'.