How to reset toolbar when disappear from Catia V5 work bench. Why it happens?

Every toolbar has a "X" button on its upper right corner. If you select it, the toolbar will be closed.

If you  Select “View > Toolbars”(your selection)

The list of current toolbars will be displayed. Currently visible toolbars are indicated by a tick symbol to the left of the toolbar name.

From  the list, click the toolbar you want to view or hide.

You can detach toolbars from the application window border by dragging the double line to the left of the toolbar. You can drag the toolbar anywhere around the screen, then dock the toolbar in the same or in another location by dragging it onto the application window border.

To restore the original positions of the toolbar on the current workbench, 

Select “View>Customize>Toolbars>Restore position”.

Sometimes the toolbar may be hidden at the lower right corner of the window. If you see a double-arrow icon at the corner, it means more toolbar are available. Drag out the other toolbar so that more can be seen.