key fell off my keyboard

If it doesn't pop back on, then it isn't going back on. You may have to replace the entire keyboard.

usually pretty easy - but depends if the keycap or platic retaining clip has been dmaged - "big keys" like shift or space also have a metal bar.

Quick google turned up a good photo guide that may help
http://www.laptoprepair101.com/laptop/2007/03/20/key-fell-off-keyboard/

good luch

If the key fell off once it will most likely fall off again - the plastic material does wear over time - especially if it is a frequently used key. A quick serach returned ressults in the < $ 40 range for a replacment keyboard http://www.laptop-keyboard-shop.com/ibm/ibm-thinkpad-t43-keyboard - not a difficult thing to change out and it just may also make the machine look a lot nicer again. When in doubt I tend to replace hardware that could cause problems at the most inconveient moment. e.g figure yoiu had to give a presntation ot type an email real quick and no key - ouch.

depends on your desire to spend money - you can often sacrifice another key to keep you going (most common fault is a broken retaining clip)  most of the keys are identicalm and there will be keys you never use... !

it's probably broken. get a new one on ebay and search on youtube on how to replace it. usually it's not very difficult.