it's hard to say how exactly they became hidden, it could be a program you installed, ie: a toolbar. Or, worse case scenario, it might be a virus being malicious. What i would suggest though is right clicking on the area at or near your buttons and selecting customize. Then, simply drag and drop the buttons from your bar to the window that opens, then ok. Close IE, open it back up, add them back, then close it. See if that helps, it could be as simple as that. Another thing i'd go ahead and do in case this doesn't help is open up your Internet Options within IE, go all the way to the last tab, Advanced, then at the bottom where it says Reset... click that, close and restart IE, essentially that should return it to normal if all else fails. Then just be careful what you edit in the future. Hope this helps. If it doesn't, post back and i'll see if i can find another solution.

Good Luck.