Intuos3 Tablet's pen is not working (by 5 users)

My Intuos3 Tablet should be working, because a blue light is turned on when it's plugged in, [as it should,] but when I either bring its pen very close to the tablet or make contact, it should change to green to show that the tablet's registered its proximity, and it should, of course, move the mouse.
Instead, when I touch the pen to the tablet, not only does the light not switch from blue to green, but nothing happens whatsoever. The mouse doesn't move, the light stays blue, and everything acts as if the pen's not even there.

I'd very much appreciate if anyone would help me out: How do I get the pen to function as it should?