Hi vietle, and welcome to Sevenforums.
As in navigation pane, to you mean the taskbar, with all the programs on there?
If you can recreate the problem, please try this tests:
Try pressing the Windows key
and tell us if the start menu pops up.
If not, to get the taskbar back, Go to task manager (Ctrl + Shift+ Esc). Go to the 'Applications' tab. Click the 'New task...' button. Type in "explorer.exe" (without the quotes) and press enter. Tell us if the taskbar comes back up.