My stylus stopped working as well after the 10 upgrade. Wasn't recognized at all by the tablet.
Not sure why, but simply removing and re-inserting the stylus battery fixed it for me.
Still not sure if anyone has got a solution. I notice that in the Device Manager there is no "HID Compliant Pen" listed. Does that have anything to do with it?
I can see a HID Compliant Pen listed in mine. The properties says it is Microsoft dated 2006-06-21 version 10.0.10240.16384. When I try to update, it tells me this is the latest. The stylus remains non-functional. Perhaps someone can provide details of a working driver.