MacBook Air No icons no menu

I have a Macbook Air, recently updated to I think IOS X something, maybe Leopard. I recently installed firefox to use instead of safari. Worked fine until system was turned off. When rebooted, nothing on the screen, no menu, no icons. Tried starting in safe mode...nothing. I don't even see Apple come up. When I've tried to log off so that I can log back in with password, nothing happens. My shortcut keys don't seem to be doing anything Any suggestions?? All I really have to work with at this point is my shortcut keys. Thanks for any assistance I can get. Lynne