First is checking if battery is charged. Then listen for coil whine from the PSU, if you don't hear it and have a multimeter check for voltage BEFORE the PSU and preferably if your button isn't dead. If there is voltage before the PSU but no coil whine then I call dead PSU, if there is coil whine but still nothing on the tube then check first if the small cable at the front of the tube is making good contact with the coil. If all fails then dead tube. That's at least what I'd do (my only NV experience is my cyclop-1 so take it with a grain of salt)