I assume you have rebooted.
After dropping my tablet onto a tile floor from about chest high (which it survived quite well I might add, thanks in part to the sacrifice of my 802.11b PC Card blade antenna!) I noticed my "Screen Rotation" pen button not working. I tapped around the button icon in increasingly larger circles until I discovered the "Sweet Spot" had drifted down and to the right about 4-5 millimeters.
Tap around those harware icons a little bit - see what happens. Otherwise, get ready to test out the Compaq Support System.