If your subwoofer is not powering on, first check that your power setting on the back is set correctly. You often have a choice between Auto (turns on after sending a signal) On (always on) or Trigger (must be connected to a receiver/preamp with a trigger cable) and if there is no choice it will be Auto On.
If your subwoofer setting is set to Auto and is not coming on, try taking the cable out of your receiver/preamp and touching the tip with your thumb. Your body generates a signal, enough to turn on the subwoofer and produce a 60Hz hum.
If your subwoofer is set to trigger, try changing it to Auto or On.
Check to see if there's a mains power switch near the AC input. If there is, make sure the mains switch is also set to on.
If your subwoofer continues to not power on, there is likely an issue with the amplifier that requires it to be either repaired or replaced depending on the model and age of the subwoofer – in this case, please contact our technical support team.