A couple of things to check...
First go to control panel, printers and other hardware, tablet and pen settings and then click the tablet buttons tab. If the button got reassigned...this is where you can reset it. Check both the primary landscape and primary portrait settings.
Second, there should be an icon in your system tray with the tool tip "Tablet Button Enable". Right click on that and make sure Tablet Button Enable is checked.
If neither of those work...perhaps it is a hardware problem.