First try connecting the power nozzle directly to the hose. If it works, the problem is due to a broken wire or faulty connection in the wands. At least one wand will need to be replaced.
My guess is it's probably due to a broken or disconnected wire. This could be in the hose (most likely), the wands, or the tank wiring itself. You can test the hose wiring by using a multimeter or continuity tester. A tester may be easier for you to use, and will provide the info needed.
Disconnect both ends of the hose, sit down, and lay the ends on your lap. Touch one end of the probe to a terminal on one end of the hose. Place other probe on a connector at the other end. One connector will light, the other won't. Now repeat with the other terminal at the first end of the hose and repeat. The terminals will read opposite the reading in the first test. If the light fails to turn on with either test, it's a broken wire in the hose. This is most likely near/at the handle end, or at the wall end. In some cases it can be repaired, but not always.
It could also be due to a faulty switch.
For a definite diagnosis, take the unit, the hose, power nozzle etc. to a repair center.