does the amp turn on, is there just nothing comming from the sub?
If the amp looks like it's working I would take the driver out of the box. Make sure it's connected. If it is then I would look at the wires that go from the cone of the sub, to the speaker terminals on the sub, and just touch them a little bit.
My brothers HSU VTF-2 had stoped working, so I took out the driver and attached it to my car audio system. And I still got no sound. Then I moved the wires going from the speaker terminals on the driver going to the cone, and then it started working.
Michael.. Same thing happened to me one time. I was checking connections, was the sub plugged in, moved the sub around, spent lots of hours troubleshooting.
Turned out someone was playing with my speaker setup on my amp/pre-pro and turned the sub off in the configuration. I answered YES to subwoofer, the rest is history.
A simple fix was the answer in my situation and I hope you have the same experience.