I'm alittle confused. By "nothing will read out" do you mean that the screen is completlely blank, or that digits are there, but no registering your speed?
If it's completely blank the only thing that can cause that is the battery. It's either dead or installed upside down. I believe they're + side up (but don't quote me). If the battery don't fix dat problem, your 'puter's busted dood.
If it's not registering your speed it's usually magnet placement. Try to get the magnet closer to the sensor either by sliding the magnet (and sensor) towards the hub (where the spokes are closer to the fork) and or shimming the sensor closer to the magnet. Also the magnet on the sigma can be rotated around the spoke. Rotate it so that it's as close to the sensor as possible.