What do you do if your tablet pen stops working?

Open Control Panel, click "System", click "Hardware", click "Device Manager". Usually, one of the devices under "Human Interface Devices", such as "HID-Compliant device" will have a warning on it. This means that it is not functioning properly.

Right-click that device, click "Uninstall". Then right-click on the heading for that device, and click "Scan for hardware changes". Your table will find the device and install it for the one that was a problem, and then continue to install others that were screwed up as well.

Sometimes the device that it re-installs will be non-functioning still. Simply repeat the process. In this case, everytime for me at least, the device will be perfect the 3rd time around.