On my Windows 2000 workstation, occasionally, the tablet just stops working. It seems that this usually happens when the mouse (which is also still connected) has been moved accidentally. However sometimes it seems to just "happen" and I am not certain why.

This knowledgebase article talks about how the driver process can be interrupted:

My tablet was working for a while, then it stopped working. Now it moves like a mouse not a tablet. What do I do?