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You may have to pull the screen for the bar to appear. This will open a list of the last five tabs that you've closed.  Note: This requires iOS 8, or iOS 7 if you're using an iPad. If you're using an older version of iOS, you'll need to use the History list in your Bookmarks to open old tabs. If need to open an older tab, you'll need to go through the History menu.
Tap the Tabs button in the bottom row of buttons. Press and hold the "+" button. Tap the tab that you want to open.