4. Have you checked the clearomizer isn't blocked?
If you have checked that your battery is turned on, and that the battery button is illuminating when pressed, the next thing to check is the clearomizer (tank/e-liquid chamber) isn't blocked.
Your e-cigarette will not work if any e-liquid has found its way down the narrow centre pipe of the clearomizer chamber. Even the most experienced of vapers experience this issue on a regular basis.
It's always a good idea to check for blockage in this pipe, so try the following remedy: unscrew the clearomizer, place a tissue where the battery was and blow into the mouth piece a couple of times. If there was any e-liquid in the centre pipe this should remove most of it. Now screw it back on and try your e-cigarette again.