At the moment,the main tool-bar is on the middle of the screen and there is no space on the tool-bar click-able to get it back to the stretched position
If by the "main toolbar" you mean the top panel, try this: Alt-F2, then type gconf-editor in the textbox and execute that command (or run it from a terminal).
In the GConf editor navigate to /apps/panel/toplevels/top_panel_screen0 (where "/" indicates a level in the tree).
Under that you'll find a setting named "expand", and if you tick the checkbox next to it, the panel should be stretched to the whole width of the screen again.
To reset your gnome panels to "factory default" delete (or better move to have a backup) the settings folder
To delete enter a terminal an type
If you log out and back in it should be ok again...