Under not circumstances should you keep a bottle of whisky on its side! No wonder your cork snapped if it was stored on its side for a year. Unlike wine, whisky should be stored upright - the higher alcoholic content of whisky causes cork to degrade. If it's left in direct contact for a long time it can even taint the flavour of a whisky. Also, as the cork begins to degrade (or of it was poorly fitting in the first place) if the whisky reaches the glue holding the cork to the plastic topper it can begin to break that down, leaching the chemicals back into the whisky.
Never, ever store whisky on its side.
Also, if you've got an open bottle whose cork looks as if it is beginning to dry out don't be tempted to wet it with the whisky, this will only speed up the drying out process.
With regards to evaporation - yes, unopened whisky will evaporate over time if it isn't properly stored. If it's stored somewhere where there are large temperature variances the cork can begin to dry out, breaking the seal and causing evaporation. You should also keep it out of direct sunlight, keep it in its box or tube etc.
The worst thing is, the use of cork is completely unnecessary. Bugger all wrong with screw tops!