Don't panic! It's simple - there's this little something we call the "pin trick".
If you look into the screw end of your cartomizer - where the cartomizer connects to the battery - you will see two components:
The threading and inner walls
A flat, circular, raised "Pin" or "Post" - most easily recognized as 'What the air hole is in the center of'.
Now that you have identified the Pin (or Post, if your prefer) I will point one of its aspects that is not so easily noticeable - It is not stationary.
In fact it can be easily pulled or PUSHED down into the threading area.
The Pin is quite an important component - it is what receives the electrical charge from the battery in order to heat the filament and vaporize the eliquid inside your cartomizer.
And so, it sometimes happens that in our daily removing and replacing Cartomizers on and off our batteries - we accidently manage to push the pin further into the threading area than can be reached by the Post on the battery - therefore rendering our LeCig vaporless.
The solution couldn't be simpler either - you need simply to take a push pin or something with a similar tip on it (even a long fingernail will do) and GENTLY run the tip underneath the edge of the Cartomizer Pin in a circular motion, thereby lifting the pin back to it's proper place and allowing you to continue vaping in style.