Why does my MacBook Air suddenly behave as if a SHIFT key is pushed down?

1. Today, suddenly, I cannot type any lowercase letters and numbers on my MacBook Air
2. The situation is just like a SHIFT key is being pushed down
3. I rebooted MacBook Air
4. Now I cannot log in to it because my password includes lowercase alphabets and numbers.
5. No information by internet search helps so far

What should I do?