Why my Macbook Air can ONLY type Capital letters and Symbols on the number buttons???

My Mac Air was still fine last night, but this morning when I tried to turn it on, it showed my password was incorrect. When I tried to reset my password I found out that my Mac Air only can type capital letters and the symbols on the number buttons. The cap lock is not on, and I didn't press the shift button. Even I tried to press the shift button, it still only had capital letters. I tried to reinstall the system. Pressed command + R when turning on, but it asked me to enter the password. There is no way to enter the correct password because only I can type are capital letters. I have no idea how to deal with it. Somebody help, please!!!!! I don't remember what my current system exactly is, but I think it should be the newest version of OS X Mountain Lion.