It can depend a bit by browser, but if your menu bar is hidden, you can usually access it with an alt code for one of the menus. Try Alt+F which is a pretty safe bet because there's almost always a menu labeled 'File.' When the menu comes up, the little underlined letters in each menu title correspond to the letter to use with Alt to bring it up.
From there you should be able to browse around through the menus, like maybe there's one called view or something, and then a toolbars option, which would let you always show it instead of hiding it. But it varies by browser so I can't say for sure.
Also one of the Function keys at the top probably toggles it as well but I don't know which one.
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