The first thing i would try is to open any folder at all. Right click on any unused part of the tool bar, and hopefully the menu that pops up will have an option 'customise' at the botton. left click on 'customise'. a new window should open.
There should be a pane on the left 'avaliable buttons' and one on the right 'current buttons'.
If the 'go back' button is in the list on the left (you may need to scroll up or down the list to find it), left click on it, and drag it into the pane on the right, and drop it where you want it.