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
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...
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.