King Reza: I know of two scenarios in which the taskbar disappears:

a) Explorer crashes
b) It is hidden

If explorer crashes, it usually just restarts. and if the taskbar is hidden, it usually reappears when you move your mouse to the bottom of the desktop. You can hide/unhide the Taskbar through Start -> Setting -> Taskbar&Startmenu.

If your taskbar is not hidden, and explorer doesn't recover, go into the task manager (Ctrl+Alt+Del -> Task Manager) and start explorer (click "new task" and type "explorer", then hit enter)