Chances are the keyboard interface cable has worked loose or is damaged.
You could try lifting out the keyboard & checking the cable/connections. The cable is usually a thin flat cable which is inserted into a plastic connector.
If you follow the link below, it will take you to the service manual for the Thinkpad T23. This tells you how to take the thing apart safely.
Failing that, you might be able to work around the problem by attaching a normal PS2 keyboard to the PS2 port on the rear of the laptop.