Would that be a DSE 901????
Regardless, first suggestion is make sure battery has power and is charged (green light while on charger)
Next, make sure atty is fully screwed on and making contact with the battery.
Then, drop 2-3 drops of juice on to the atty and try vaping. If using just carts, make sure there cart filling is wet/juicey and not dry or stuffed all the way down.
If still nothing, it is possible the atty and/or battery is dead - it's sad but happens.