If everything you type is in capital letters, there are two things you'll want to check. First, check to see if your caps lock is on - there should be a light that's turned on if it is. Also, check to see if either shift button isn't pressed down because of gunk or buildup in the keyboard itself.
The most probable cause is that you have inadvertantly hit the caps lock key. Just press it again and the problem will be solved. If the caps lock is off try to restart your computer as turning electronics off and back on usually resets it. If the problem persists take the keyboard to a repair shop