Because you must of pressed F1, to get your HUD back press F1
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it means the server has gone down such as minecraft server has crashed or the server owner has lost Internet connection
You can't play minecraft alpha 1.2.6. You can currently play minecraft beta 1.8 if you buy the game for $21.95 and then play it in the browser or download it to your computer and play it in an application. Alpha 1.2.6 is long gone, and it is not able to be found anywhere, as far as I know.
HUD means Heads Up Display.
Check the recycle bin, if it's in there press "restore" if not, then it's gone...