Do you have power going the screen? Is there a usb port? If there is, and you plug it in, does the display show anything?
So say you have no power at all and can't get anything to show on the screen.. mod is probably dead.
If you at least have a display but your mod will not fire, it is most likely that the small on off pin underneath the fire button is dirty and not making proper contact. Sometimes you can clean it with a slightly damp q-tip if you can get the outer case off and most often, you end up either replacing the mod or finding someone that can solder a new mini switch to the circuit board.
One of the things I like about my Segelei is that the power button is far away from the 510 connector and atomizer.. so if i get a leak, it is harder to get inside my mod.