All of them are closed? If you still have one open you can right click on an empty gem or on the border and click Open new hotbar.
If not try going into your Options window, under User Interface (make sure you hit the Advanced button on the top right of the options window).
Find the option under Game Windows -> Always Show Threat Window and uncheck it and recheck it again. see if that gets you at least one hotbar back.
Otherwise try:
/show_window MainHUD.Hotkey
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