The ribbon cable may have come loose.
Slide the latches over to unlock the battery.
Remove the battery.
Turn the laptop over and open it up.
Using a small flat head screwdriver, press in the keyboard latches and carefully remove and turn over the keyboard.
You will then see the keyboard ribbon connector.
Check that the ribbon is securely fastened below the catch. You may want to gently remove it, and re-insert it to ensure no dirt has got in.
Reassemble in the reverse order.
This may or may not cure the problem, but this is where i would start!