Sounds like you've accidentally switched to an English (US) keyboard,
perhaps by typing LeftAlt+Shift, or perhaps because your computer
(laptops frequently are this way) has built-in international support,
and you triggered a different keyboard.
If the former, LeftAlt+Shift will toggle/cycle through all the
keyboards that are installed. If you won't ever need a US keyboard,
then (in Windows) go to Start > Control Panel > Regional and Language
Options, and click through until you get to the "Remove Keyboards"
area (it works rather differently in XP and Vista). If you want it
back someday, just go there again and reinstate it.